logo 30 seconds of code Curated collection of useful JavaScript snippets that you can understand in 30 seconds or less.

 

Adapter

call

Given a key and a set of arguments, call them when given a context. Primarily useful in composition.

Use a closure to call a stored key with stored arguments.

const call = (key, ...args) => context => context[key](...args);
Promise.resolve([1, 2, 3])
  .then(call('map', x => 2 * x))
  .then(console.log); //[ 2, 4, 6 ]
const map = call.bind(null, 'map');
Promise.resolve([1, 2, 3])
  .then(map(x => 2 * x))
  .then(console.log); //[ 2, 4, 6 ]

collectInto

Changes a function that accepts an array into a variadic function.

Given a function, return a closure that collects all inputs into an array-accepting function.

const collectInto = fn => (...args) => fn(args);
const Pall = collectInto(Promise.all.bind(Promise));
let p1 = Promise.resolve(1);
let p2 = Promise.resolve(2);
let p3 = new Promise(resolve => setTimeout(resolve, 2000, 3));
Pall(p1, p2, p3).then(console.log);

flip

Flip takes a function as an argument, then makes the first argument the last.

Return a closure that takes variadic inputs, and splices the last argument to make it the first argument before applying the rest.

const flip = fn => (first, ...rest) => fn(...rest, first);
let a = { name: 'John Smith' };
let b = {};
const mergeFrom = flip(Object.assign);
let mergePerson = mergeFrom.bind(null, a);
mergePerson(b); // == b
b = {};
Object.assign(b, a); // == b

pipeFunctions

Performs left-to-right function composition.

Use Array.reduce() with the spread operator (...) to perform left-to-right function composition. The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.

const pipeFunctions = (...fns) => fns.reduce((f, g) => (...args) => g(f(...args)));
const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = pipeFunctions(multiply, add5);
multiplyAndAdd5(5, 2); // 15

promisify

Converts an asynchronous function to return a promise.

Use currying to return a function returning a Promise that calls the original function. Use the ...rest operator to pass in all the parameters.

In Node 8+, you can use util.promisify

const promisify = func => (...args) =>
  new Promise((resolve, reject) =>
    func(...args, (err, result) => (err ? reject(err) : resolve(result)))
  );
const delay = promisify((d, cb) => setTimeout(cb, d));
delay(2000).then(() => console.log('Hi!')); // // Promise resolves after 2s

spreadOver

Takes a variadic function and returns a closure that accepts an array of arguments to map to the inputs of the function.

Use closures and the spread operator (...) to map the array of arguments to the inputs of the function.

const spreadOver = fn => argsArr => fn(...argsArr);
const arrayMax = spreadOver(Math.max);
arrayMax([1, 2, 3]); // 3

Array

chunk

Chunks an array into smaller arrays of a specified size.

Use Array.from() to create a new array, that fits the number of chunks that will be produced. Use Array.slice() to map each element of the new array to a chunk the length of size. If the original array can't be split evenly, the final chunk will contain the remaining elements.

const chunk = (arr, size) =>
  Array.from({ length: Math.ceil(arr.length / size) }, (v, i) =>
    arr.slice(i * size, i * size + size)
  );
chunk([1, 2, 3, 4, 5], 2); // [[1,2],[3,4],[5]]

compact

Removes falsey values from an array.

Use Array.filter() to filter out falsey values (false, null, 0, "", undefined, and NaN).

const compact = arr => arr.filter(Boolean);
compact([0, 1, false, 2, '', 3, 'a', 'e' * 23, NaN, 's', 34]); // [ 1, 2, 3, 'a', 's', 34 ]

countBy

Groups the elements of an array based on the given function and returns the count of elements in each group.

Use Array.map() to map the values of an array to a function or property name. Use Array.reduce() to create an object, where the keys are produced from the mapped results.

const countBy = (arr, fn) =>
  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
    acc[val] = (acc[val] || 0) + 1;
    return acc;
  }, {});
countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2}
countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1}

countOccurrences

Counts the occurrences of a value in an array.

Use Array.reduce() to increment a counter each time you encounter the specific value inside the array.

const countOccurrences = (arr, val) => arr.reduce((a, v) => (v === val ? a + 1 : a + 0), 0);
countOccurrences([1, 1, 2, 1, 2, 3], 1); // 3

deepFlatten

Deep flattens an array.

Use recursion. Use Array.concat() with an empty array ([]) and the spread operator (...) to flatten an array. Recursively flatten each element that is an array.

const deepFlatten = arr => [].concat(...arr.map(v => (Array.isArray(v) ? deepFlatten(v) : v)));
deepFlatten([1, [2], [[3], 4], 5]); // [1,2,3,4,5]

difference

Returns the difference between two arrays.

Create a Set from b, then use Array.filter() on a to only keep values not contained in b.

const difference = (a, b) => {
  const s = new Set(b);
  return a.filter(x => !s.has(x));
};
difference([1, 2, 3], [1, 2, 4]); // [3]

differenceWith

Filters out all values from an array for which the comparator function does not return true.

Use Array.filter() and Array.findIndex() to find the appropriate values.

const differenceWith = (arr, val, comp) => arr.filter(a => val.findIndex(b => comp(a, b)) === -1);
differenceWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2]

dropElements

Removes elements in an array until the passed function returns true. Returns the remaining elements in the array.

Loop through the array, using Array.slice() to drop the first element of the array until the returned value from the function is true. Returns the remaining elements.

const dropElements = (arr, func) => {
  while (arr.length > 0 && !func(arr[0])) arr = arr.slice(1);
  return arr;
};
dropElements([1, 2, 3, 4], n => n >= 3); // [3,4]

dropRight

Returns a new array with n elements removed from the right.

Use Array.slice() to slice the remove the specified number of elements from the right.

const dropRight = (arr, n = 1) => arr.slice(0, -n);
dropRight([1, 2, 3]); // [1,2]
dropRight([1, 2, 3], 2); // [1]
dropRight([1, 2, 3], 42); // []

everyNth

Returns every nth element in an array.

Use Array.filter() to create a new array that contains every nth element of a given array.

const everyNth = (arr, nth) => arr.filter((e, i) => i % nth === nth - 1);
everyNth([1, 2, 3, 4, 5, 6], 2); // [ 2, 4, 6 ]

filterNonUnique

Filters out the non-unique values in an array.

Use Array.filter() for an array containing only the unique values.

const filterNonUnique = arr => arr.filter(i => arr.indexOf(i) === arr.lastIndexOf(i));
filterNonUnique([1, 2, 2, 3, 4, 4, 5]); // [1,3,5]

findLast

Returns the last element for which the provided function returns a truthy value.

Use Array.filter() to remove elements for which fn returns falsey values, Array.slice(-1) to get the last one.

const findLast = (arr, fn) => arr.filter(fn).slice(-1);
findLast([1, 2, 3, 4], n => n % 2 === 1); // 3

flatten

Flattens an array up to the specified depth.

Use recursion, decrementing depth by 1 for each level of depth. Use Array.reduce() and Array.concat() to merge elements or arrays. Base case, for depth equal to 1 stops recursion. Omit the second argument, depth to flatten only to a depth of 1 (single flatten).

const flatten = (arr, depth = 1) =>
  depth != 1
    ? arr.reduce((a, v) => a.concat(Array.isArray(v) ? flatten(v, depth - 1) : v), [])
    : arr.reduce((a, v) => a.concat(v), []);
flatten([1, [2], 3, 4]); // [1, 2, 3, 4]
flatten([1, [2, [3, [4, 5], 6], 7], 8], 2); // [1, 2, 3, [4, 5], 6, 7, 8]

forEachRight

Executes a provided function once for each array element, starting from the array's last element.

Use Array.slice(0) to clone the given array, Array.reverse() to reverse it and Array.forEach() to iterate over the reversed array.

const forEachRight = (arr, callback) =>
  arr
    .slice(0)
    .reverse()
    .forEach(callback);
forEachRight([1, 2, 3, 4], val => console.log(val)); // '4', '3', '2', '1'

groupBy

Groups the elements of an array based on the given function.

Use Array.map() to map the values of an array to a function or property name. Use Array.reduce() to create an object, where the keys are produced from the mapped results.

const groupBy = (arr, fn) =>
  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
    acc[val] = (acc[val] || []).concat(arr[i]);
    return acc;
  }, {});
groupBy([6.1, 4.2, 6.3], Math.floor); // {4: [4.2], 6: [6.1, 6.3]}
groupBy(['one', 'two', 'three'], 'length'); // {3: ['one', 'two'], 5: ['three']}

Returns the head of a list.

Use arr[0] to return the first element of the passed array.

const head = arr => arr[0];
head([1, 2, 3]); // 1

indexOfAll

Returns all indices of val in an array. If val never occurs, returns [].

Use Array.forEach() to loop over elements and Array.push() to store indices for matching elements. Return the array of indices.

const indexOfAll = (arr, val) => {
  const indices = [];
  arr.forEach((el, i) => el === val && indices.push(i));
  return indices;
};
indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0,3]
indexOfAll([1, 2, 3], 4); // []

initial

Returns all the elements of an array except the last one.

Use arr.slice(0,-1) to return all but the last element of the array.

const initial = arr => arr.slice(0, -1);
initial([1, 2, 3]); // [1,2]

initialize2DArray

Initializes a 2D array of given width and height and value.

Use Array.map() to generate h rows where each is a new array of size w initialize with value. If the value is not provided, default to null.

const initialize2DArray = (w, h, val = null) =>
  Array.from({ length: h }).map(() => Array.from({ length: w }).fill(val));
initialize2DArray(2, 2, 0); // [[0,0], [0,0]]

initializeArrayWithRange

Initializes an array containing the numbers in the specified range where start and end are inclusive with their common difference step.

Use Array.from(Math.ceil((end+1-start)/step)) to create an array of the desired length(the amounts of elements is equal to (end-start)/step or (end+1-start)/step for inclusive end), Array.map() to fill with the desired values in a range. You can omit start to use a default value of 0. You can omit step to use a default value of 1.

const initializeArrayWithRange = (end, start = 0, step = 1) =>
  Array.from({ length: Math.ceil((end + 1 - start) / step) }).map((v, i) => i * step + start);
initializeArrayWithRange(5); // [0,1,2,3,4,5]
initializeArrayWithRange(7, 3); // [3,4,5,6,7]
initializeArrayWithRange(9, 0, 2); // [0,2,4,6,8]

initializeArrayWithRangeRight

Initializes an array containing the numbers in the specified range (in reverse) where start and end are inclusive with their common difference step.

Use Array.from(Math.ceil((end+1-start)/step)) to create an array of the desired length(the amounts of elements is equal to (end-start)/step or (end+1-start)/step for inclusive end), Array.map() to fill with the desired values in a range. You can omit start to use a default value of 0. You can omit step to use a default value of 1.

const initializeArrayWithRangeRight = (end, start = 0, step = 1) =>
  Array.from({ length: Math.ceil((end + 1 - start) / step) }).map(
    (v, i, arr) => (arr.length - i - 1) * step + start
  );
initializeArrayWithRangeRight(5); // [5,4,3,2,1,0]
initializeArrayWithRangeRight(7, 3); // [7,6,5,4,3]
initializeArrayWithRangeRight(9, 0, 2); // [8,6,4,2,0]

initializeArrayWithValues

Initializes and fills an array with the specified values.

Use Array(n) to create an array of the desired length, fill(v) to fill it with the desired values. You can omit val to use a default value of 0.

const initializeArrayWithValues = (n, val = 0) => Array(n).fill(val);
initializeArrayWithValues(5, 2); // [2,2,2,2,2]

intersection

Returns a list of elements that exist in both arrays.

Create a Set from b, then use Array.filter() on a to only keep values contained in b.

const intersection = (a, b) => {
  const s = new Set(b);
  return a.filter(x => s.has(x));
};
intersection([1, 2, 3], [4, 3, 2]); // [2,3]

isSorted

Returns 1 if the array is sorted in ascending order, -1 if it is sorted in descending order or 0 if it is not sorted.

Calculate the ordering direction for the first two elements. Use Object.entries() to loop over array objects and compare them in pairs. Return 0 if the direction changes or the direction if the last element is reached.

const isSorted = arr => {
  const direction = arr[0] > arr[1] ? -1 : 1;
  for (let [i, val] of arr.entries())
    if (i === arr.length - 1) return direction;
    else if ((val - arr[i + 1]) * direction > 0) return 0;
};
isSorted([0, 1, 2, 2]); // 1
isSorted([4, 3, 2]); // -1
isSorted([4, 3, 5]); // 0

join

Joins all elements of an array into a string and returns this string. Uses a separator and an end separator.

Use Array.reduce() to combine elements into a string. Omit the second argument, separator, to use a default separator of ','. Omit the third argument, end, to use the same value as separator by default.

const join = (arr, separator = ',', end = separator) =>
  arr.reduce(
    (acc, val, i) =>
      i == arr.length - 2
        ? acc + val + end
        : i == arr.length - 1 ? acc + val : acc + val + separator,
    ''
  );
join(['pen', 'pineapple', 'apple', 'pen'], ',', '&'); // "pen,pineapple,apple&pen"
join(['pen', 'pineapple', 'apple', 'pen'], ','); // "pen,pineapple,apple,pen"
join(['pen', 'pineapple', 'apple', 'pen']); // "pen,pineapple,apple,pen"

last

Returns the last element in an array.

Use arr.length - 1 to compute the index of the last element of the given array and returning it.

const last = arr => arr[arr.length - 1];
last([1, 2, 3]); // 3

longestItem

Takes any number of iterable objects or objects with a length property and returns the longest one.

Use Array.sort() to sort all arguments by length, return the first (longest) one.

const longestItem = (...vals) => [...vals].sort((a, b) => b.length - a.length)[0];
longestItem('this', 'is', 'a', 'testcase'); // 'testcase'
longestItem(...['a', 'ab', 'abc']); // 'abc'
longestItem(...['a', 'ab', 'abc'], 'abcd'); // 'abcd'
longestItem([1, 2, 3], [1, 2], [1, 2, 3, 4, 5]); // [1, 2, 3, 4, 5]
longestItem([1, 2, 3], 'foobar'); // 'foobar'

mapObject

Maps the values of an array to an object using a function, where the key-value pairs consist of the original value as the key and the mapped value.

Use an anonymous inner function scope to declare an undefined memory space, using closures to store a return value. Use a new Array to store the array with a map of the function over its data set and a comma operator to return a second step, without needing to move from one context to another (due to closures and order of operations).

const mapObject = (arr, fn) =>
  (a => (
    (a = [arr, arr.map(fn)]), a[0].reduce((acc, val, ind) => ((acc[val] = a[1][ind]), acc), {})
  ))();
const squareIt = arr => mapObject(arr, a => a * a);
squareIt([1, 2, 3]); // { 1: 1, 2: 4, 3: 9 }

maxN

Returns the n maximum elements from the provided array. If n is greater than or equal to the provided array's length, then return the original array(sorted in descending order).

Use Array.sort() combined with the spread operator (...) to create a shallow clone of the array and sort it in descending order. Use Array.slice() to get the specified number of elements. Omit the second argument, n, to get a one-element array.

const maxN = (arr, n = 1) => [...arr].sort((a, b) => b - a).slice(0, n);
maxN([1, 2, 3]); // [3]
maxN([1, 2, 3], 2); // [3,2]

minN

Returns the n minimum elements from the provided array. If n is greater than or equal to the provided array's length, then return the original array(sorted in ascending order).

Use Array.sort() combined with the spread operator (...) to create a shallow clone of the array and sort it in ascending order. Use Array.slice() to get the specified number of elements. Omit the second argument, n, to get a one-element array.

const minN = (arr, n = 1) => [...arr].sort((a, b) => a - b).slice(0, n);
minN([1, 2, 3]); // [1]
minN([1, 2, 3], 2); // [1,2]

nthElement

Returns the nth element of an array.

Use Array.slice() to get an array containing the nth element at the first place. If the index is out of bounds, return []. Omit the second argument, n, to get the first element of the array.

const nthElement = (arr, n = 0) => (n > 0 ? arr.slice(n, n + 1) : arr.slice(n))[0];
nthElement(['a', 'b', 'c'], 1); // 'b'
nthElement(['a', 'b', 'b'], -3); // 'a'

partition

Groups the elements into two arrays, depending on the provided function's truthiness for each element.

Use Array.reduce() to create an array of two arrays. Use Array.push() to add elements for which fn returns true to the first array and elements for which fn returns false to the second one.

const partition = (arr, fn) =>
  arr.reduce(
    (acc, val, i, arr) => {
      acc[fn(val, i, arr) ? 0 : 1].push(val);
      return acc;
    },
    [[], []]
  );
const users = [{ user: 'barney', age: 36, active: false }, { user: 'fred', age: 40, active: true }];
partition(users, o => o.active); // [[{ 'user': 'fred',    'age': 40, 'active': true }],[{ 'user': 'barney',  'age': 36, 'active': false }]]

pull

Mutates the original array to filter out the values specified.

Use Array.filter() and Array.includes() to pull out the values that are not needed. Use Array.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.push() to re-populate it with only the pulled values.

(For a snippet that does not mutate the original array see without)

const pull = (arr, ...args) => {
  let argState = Array.isArray(args[0]) ? args[0] : args;
  let pulled = arr.filter((v, i) => !argState.includes(v));
  arr.length = 0;
  pulled.forEach(v => arr.push(v));
};
let myArray = ['a', 'b', 'c', 'a', 'b', 'c'];
pull(myArray, 'a', 'c'); // myArray = [ 'b', 'b' ]

pullAtIndex

Mutates the original array to filter out the values at the specified indexes.

Use Array.filter() and Array.includes() to pull out the values that are not needed. Use Array.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.push() to re-populate it with only the pulled values. Use Array.push() to keep track of pulled values

const pullAtIndex = (arr, pullArr) => {
  let removed = [];
  let pulled = arr
    .map((v, i) => (pullArr.includes(i) ? removed.push(v) : v))
    .filter((v, i) => !pullArr.includes(i));
  arr.length = 0;
  pulled.forEach(v => arr.push(v));
  return removed;
};
let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtIndex(myArray, [1, 3]); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]

pullAtValue

Mutates the original array to filter out the values specified. Returns the removed elements.

Use Array.filter() and Array.includes() to pull out the values that are not needed. Use Array.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.push() to re-populate it with only the pulled values. Use Array.push() to keep track of pulled values

const pullAtValue = (arr, pullArr) => {
  let removed = [],
    pushToRemove = arr.forEach((v, i) => (pullArr.includes(v) ? removed.push(v) : v)),
    mutateTo = arr.filter((v, i) => !pullArr.includes(v));
  arr.length = 0;
  mutateTo.forEach(v => arr.push(v));
  return removed;
};
let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtValue(myArray, ['b', 'd']); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]

reducedFilter

Filter an array of objects based on a condition while also filtering out unspecified keys.

Use Array.filter() to filter the array based on the predicate fn so that it returns the objects for which the condition returned a truthy value. On the filtered array, use Array.map() to return the new object using Array.reduce() to filter out the keys which were not supplied as the keys argument.

const reducedFilter = (data, keys, fn) =>
  data.filter(fn).map(el =>
    keys.reduce((acc, key) => {
      acc[key] = el[key];
      return acc;
    }, {})
  );
const data = [
  {
    id: 1,
    name: 'john',
    age: 24
  },
  {
    id: 2,
    name: 'mike',
    age: 50
  }
];

reducedFilter(data, ['id', 'name'], item => item.age > 24); // [{ id: 2, name: 'mike'}]

remove

Removes elements from an array for which the given function returns false.

Use Array.filter() to find array elements that return truthy values and Array.reduce() to remove elements using Array.splice(). The func is invoked with three arguments (value, index, array).

const remove = (arr, func) =>
  Array.isArray(arr)
    ? arr.filter(func).reduce((acc, val) => {
        arr.splice(arr.indexOf(val), 1);
        return acc.concat(val);
      }, [])
    : [];
remove([1, 2, 3, 4], n => n % 2 == 0); // [2, 4]

sample

Returns a random element from an array.

Use Math.random() to generate a random number, multiply it by length and round it of to the nearest whole number using Math.floor(). This method also works with strings.

const sample = arr => arr[Math.floor(Math.random() * arr.length)];
sample([3, 7, 9, 11]); // 9

sampleSize

Gets n random elements at unique keys from array up to the size of array.

Shuffle the array using the Fisher-Yates algorithm. Use Array.slice() to get the first n elements. Omit the second argument, n to get only one element at random from the array.

const sampleSize = ([...arr], n = 1) => {
  let m = arr.length;
  while (m) {
    const i = Math.floor(Math.random() * m--);
    [arr[m], arr[i]] = [arr[i], arr[m]];
  }
  return arr.slice(0, n);
};
sampleSize([1, 2, 3], 2); // [3,1]
sampleSize([1, 2, 3], 4); // [2,3,1]

shuffle

Randomizes the order of the values of an array, returning a new array.

Uses the Fisher-Yates algorithm to reorder the elements of the array.

const shuffle = ([...arr]) => {
  let m = arr.length;
  while (m) {
    const i = Math.floor(Math.random() * m--);
    [arr[m], arr[i]] = [arr[i], arr[m]];
  }
  return arr;
};
const foo = [1, 2, 3];
shuffle(foo); // [2,3,1], foo = [1,2,3]

similarity

Returns an array of elements that appear in both arrays.

Use Array.filter() to remove values that are not part of values, determined using Array.includes().

const similarity = (arr, values) => arr.filter(v => values.includes(v));
similarity([1, 2, 3], [1, 2, 4]); // [1,2]

sortedIndex

Returns the lowest index at which value should be inserted into array in order to maintain its sort order.

Check if the array is sorted in descending order (loosely). Use Array.findIndex() to find the appropriate index where the element should be inserted.

const sortedIndex = (arr, n) => {
  const isDescending = arr[0] > arr[arr.length - 1];
  const index = arr.findIndex(el => (isDescending ? n >= el : n <= el));
  return index === -1 ? arr.length : index;
};
sortedIndex([5, 3, 2, 1], 4); // 1
sortedIndex([30, 50], 40); // 1

symmetricDifference

Returns the symmetric difference between two arrays.

Create a Set from each array, then use Array.filter() on each of them to only keep values not contained in the other.

const symmetricDifference = (a, b) => {
  const sA = new Set(a),
    sB = new Set(b);
  return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))];
};
symmetricDifference([1, 2, 3], [1, 2, 4]); // [3,4]

tail

Returns all elements in an array except for the first one.

Return Array.slice(1) if the array's length is more than 1, otherwise, return the whole array.

const tail = arr => (arr.length > 1 ? arr.slice(1) : arr);
tail([1, 2, 3]); // [2,3]
tail([1]); // [1]

take

Returns an array with n elements removed from the beginning.

Use Array.slice() to create a slice of the array with n elements taken from the beginning.

const take = (arr, n = 1) => arr.slice(0, n);
take([1, 2, 3], 5); // [1, 2, 3]
take([1, 2, 3], 0); // []

takeRight

Returns an array with n elements removed from the end.

Use Array.slice() to create a slice of the array with n elements taken from the end.

const takeRight = (arr, n = 1) => arr.slice(arr.length - n, arr.length);
takeRight([1, 2, 3], 2); // [ 2, 3 ]
takeRight([1, 2, 3]); // [3]

union

Returns every element that exists in any of the two arrays once.

Create a Set with all values of a and b and convert to an array.

const union = (a, b) => Array.from(new Set([...a, ...b]));
union([1, 2, 3], [4, 3, 2]); // [1,2,3,4]

uniqueElements

Returns all unique values of an array.

Use ES6 Set and the ...rest operator to discard all duplicated values.

const uniqueElements = arr => [...new Set(arr)];
uniqueElements([1, 2, 2, 3, 4, 4, 5]); // [1,2,3,4,5]

without

Filters out the elements of an array, that have one of the specified values.

Use Array.filter() to create an array excluding(using !Array.includes()) all given values.

(For a snippet that mutates the original array see pull)

const without = (arr, ...args) => arr.filter(v => !args.includes(v));
without([2, 1, 2, 3], 1, 2); // [3]

zip

Creates an array of elements, grouped based on the position in the original arrays.

Use Math.max.apply() to get the longest array in the arguments. Creates an array with that length as return value and use Array.from() with a map-function to create an array of grouped elements. If lengths of the argument-arrays vary, undefined is used where no value could be found.

const zip = (...arrays) => {
  const maxLength = Math.max(...arrays.map(x => x.length));
  return Array.from({ length: maxLength }).map((_, i) => {
    return Array.from({ length: arrays.length }, (_, k) => arrays[k][i]);
  });
};
zip(['a', 'b'], [1, 2], [true, false]); // [['a', 1, true], ['b', 2, false]]
zip(['a'], [1, 2], [true, false]); // [['a', 1, true], [undefined, 2, false]]

zipObject

Given an array of valid property identifiers and an array of values, return an object associating the properties to the values.

Since an object can have undefined values but not undefined property pointers, the array of properties is used to decide the structure of the resulting object using Array.reduce().

const zipObject = (props, values) =>
  props.reduce((obj, prop, index) => ((obj[prop] = values[index]), obj), {});
zipObject(['a', 'b', 'c'], [1, 2]); // {a: 1, b: 2, c: undefined}
zipObject(['a', 'b'], [1, 2, 3]); // {a: 1, b: 2}

Browser

arrayToHtmlList

Converts the given array elements into <li> tags and appends them to the list of the given id.

Use Array.map() and document.querySelector() to create a list of html tags.

const arrayToHtmlList = (arr, listID) =>
  arr.map(item => (document.querySelector('#' + listID).innerHTML += `<li>${item}</li>`));
arrayToHtmlList(['item 1', 'item 2'], 'myListID');

bottomVisible

Returns true if the bottom of the page is visible, false otherwise.

Use scrollY, scrollHeight and clientHeight to determine if the bottom of the page is visible.

const bottomVisible = () =>
  document.documentElement.clientHeight + window.scrollY >=
  (document.documentElement.scrollHeight || document.documentElement.clientHeight);
bottomVisible(); // true

copyToClipboardadvanced

Copy a string to the clipboard. Only works as a result of user action (i.e. inside a click event listener).

Create a new <textarea> element, fill it with the supplied data and add it to the HTML document. Use Selection.getRangeAt()to store the selected range (if any). Use document.execCommand('copy') to copy to the clipboard. Remove the <textarea> element from the HTML document. Finally, use Selection().addRange() to recover the original selected range (if any).

const copyToClipboard = str => {
  const el = document.createElement('textarea');
  el.value = str;
  el.setAttribute('readonly', '');
  el.style.position = 'absolute';
  el.style.left = '-9999px';
  document.body.appendChild(el);
  const selected =
    document.getSelection().rangeCount > 0 ? document.getSelection().getRangeAt(0) : false;
  el.select();
  document.execCommand('copy');
  document.body.removeChild(el);
  if (selected) {
    document.getSelection().removeAllRanges();
    document.getSelection().addRange(selected);
  }
};
copyToClipboard('Lorem ipsum'); // 'Lorem ipsum' copied to clipboard.

createElement

Creates an element from a string (without appending it to the document). If the given string contains multiple elements, only the first one will be returned.

Use document.createElement() to create a new element. Set its innerHTML to the string supplied as the argument. Use ParentNode.firstElementChild to return the element version of the string.

const createElement = str => {
  const el = document.createElement('div');
  el.innerHTML = str;
  return el.firstElementChild;
};
const el = createElement(
  `<div class="container">
    <p>Hello!</p>
  </div>`
);
console.log(el.className); // 'container'

createEventHubadvanced

Creates a pub/sub (publish–subscribe) event hub with emit, on, and off methods.

Use Object.create(null) to create an empty hub object that does not inherit properties from Object.prototype. For emit, resolve the array of handlers based on the event argument and then run each one with Array.forEach() by passing in the data as an argument. For on, create an array for the event if it does not yet exist, then use Array.push() to add the handler to the array. For off, use Array.findIndex() to find the index of the handler in the event array and remove it using Array.splice().

const createEventHub = () => ({
  hub: Object.create(null),
  emit(event, data) {
    (this.hub[event] || []).forEach(handler => handler(data));
  },
  on(event, handler) {
    if (!this.hub[event]) this.hub[event] = [];
    this.hub[event].push(handler);
  },
  off(event, handler) {
    const i = (this.hub[event] || []).findIndex(h => h === handler);
    if (i > -1) this.hub[event].splice(i, 1);
  }
});
const handler = data => console.log(data);
const hub = createEventHub();
let increment = 0;

// Subscribe: listen for different types of events
hub.on('message', handler);
hub.on('message', () => console.log('Message event fired'));
hub.on('increment', () => increment++);

// Publish: emit events to invoke all handlers subscribed to them, passing the data to them as an argument
hub.emit('message', 'hello world'); // logs 'hello world' and 'Message event fired'
hub.emit('message', { hello: 'world' }); // logs the object and 'Message event fired'
hub.emit('increment'); // `increment` variable is now 1

// Unsubscribe: stop a specific handler from listening to the 'message' event
hub.off('message', handler);

currentURL

Returns the current URL.

Use window.location.href to get current URL.

const currentURL = () => window.location.href;
currentURL(); // 'https://google.com'

detectDeviceType

Detects wether the website is being opened in a mobile device or a desktop/laptop.

Use a regular expression to test the navigator.userAgent property to figure out if the device is a mobile device or a desktop/laptop.

const detectDeviceType = () =>
  /Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(navigator.userAgent)
    ? 'Mobile'
    : 'Desktop';
detectDeviceType(); // "Mobile" or "Desktop"

elementIsVisibleInViewport

Returns true if the element specified is visible in the viewport, false otherwise.

Use Element.getBoundingClientRect() and the window.inner(Width|Height) values to determine if a given element is visible in the viewport. Omit the second argument to determine if the element is entirely visible, or specify true to determine if it is partially visible.

const elementIsVisibleInViewport = (el, partiallyVisible = false) => {
  const { top, left, bottom, right } = el.getBoundingClientRect();
  const { innerHeight, innerWidth } = window;
  return partiallyVisible
    ? ((top > 0 && top < innerHeight) || (bottom > 0 && bottom < innerHeight)) &&
        ((left > 0 && left < innerWidth) || (right > 0 && right < innerWidth))
    : top >= 0 && left >= 0 && bottom <= innerHeight && right <= innerWidth;
};
// e.g. 100x100 viewport and a 10x10px element at position {top: -1, left: 0, bottom: 9, right: 10}
elementIsVisibleInViewport(el); // false - (not fully visible)
elementIsVisibleInViewport(el, true); // true - (partially visible)

getScrollPosition

Returns the scroll position of the current page.

Use pageXOffset and pageYOffset if they are defined, otherwise scrollLeft and scrollTop. You can omit el to use a default value of window.

const getScrollPosition = (el = window) => ({
  x: el.pageXOffset !== undefined ? el.pageXOffset : el.scrollLeft,
  y: el.pageYOffset !== undefined ? el.pageYOffset : el.scrollTop
});
getScrollPosition(); // {x: 0, y: 200}

getStyle

Returns the value of a CSS rule for the specified element.

Use Window.getComputedStyle() to get the value of the CSS rule for the specified element.

const getStyle = (el, ruleName) => getComputedStyle(el)[ruleName];
getStyle(document.querySelector('p'), 'font-size'); // '16px'

hasClass

Returns true if the element has the specified class, false otherwise.

Use element.classList.contains() to check if the element has the specified class.

const hasClass = (el, className) => el.classList.contains(className);
hasClass(document.querySelector('p.special'), 'special'); // true

hashBrowseradvanced

Creates a hash for a value using the SHA-256 algorithm. Returns a promise.

Use the SubtleCrypto API to create a hash for the given value.

const hashBrowser = val =>
  crypto.subtle.digest('SHA-256', new TextEncoder('utf-8').encode(val)).then(h => {
    let hexes = [],
      view = new DataView(h);
    for (let i = 0; i < view.byteLength; i += 4)
      hexes.push(('00000000' + view.getUint32(i).toString(16)).slice(-8));
    return hexes.join('');
  });
hashBrowser(JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })).then(console.log); // '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'

hide

Hides all the elements specified.

Use the spread operator (...) and Array.forEach() to apply display: none to each element specified.

const hide = (...el) => [...el].forEach(e => (e.style.display = 'none'));
hide(...document.querySelectorAll('img')); // Hides all <img> elements on the page

httpsRedirect

Redirects the page to HTTPS if its currently in HTTP. Also, pressing the back button doesn't take it back to the HTTP page as its replaced in the history.

Use location.protocol to get the protocol currently being used. If it's not HTTPS, use location.replace() to replace the existing page with the HTTPS version of the page. Use location.href to get the full address, split it with String.split() and remove the protocol part of the URL.

const httpsRedirect = () => {
  if (location.protocol !== 'https:') location.replace('https://' + location.href.split('//')[1]);
};
httpsRedirect(); // If you are on http://mydomain.com, you are redirected to https://mydomain.com

observeMutationsadvanced

Returns a new MutationObserver and runs the provided callback for each mutation on the specified element.

Use a MutationObserver to observe mutations on the given element. Use Array.forEach() to run the callback for each mutation that is observed. Omit the third argument, options, to use the default options (all true).

const observeMutations = (element, callback, options) => {
  const observer = new MutationObserver(mutations => mutations.forEach(m => callback(m)));
  observer.observe(
    element,
    Object.assign(
      {
        childList: true,
        attributes: true,
        attributeOldValue: true,
        characterData: true,
        characterDataOldValue: true,
        subtree: true
      },
      options
    )
  );
  return observer;
};
const obs = observeMutations(document, console.log); // Logs all mutations that happen on the page
obs.disconnect(); // Disconnects the observer and stops logging mutations on the page

off

Removes an event listener from an element.

Use EventTarget.removeEventListener() to remove an event listener from an element. Omit the fourth argument opts to use false or specify it based on the options used when the event listener was added.

const off = (el, evt, fn, opts = false) => el.removeEventListener(evt, fn, opts);
const fn = () => console.log('!');
document.body.addEventListener('click', fn);
off(document.body, 'click', fn); // no longer logs '!' upon clicking on the page

on

Adds an event listener to an element with the ability to use event delegation.

Use EventTarget.addEventListener() to add an event listener to an element. If there is a target property supplied to the options object, ensure the event target matches the target specified and then invoke the callback by supplying the correct this context. Returns a reference to the custom delegator function, in order to be possible to use with off. Omit opts to default to non-delegation behavior and event bubbling.

const on = (el, evt, fn, opts = {}) => {
  const delegatorFn = e => e.target.matches(opts.target) && fn.call(e.target, e);
  el.addEventListener(evt, opts.target ? delegatorFn : fn, opts.options || false);
  if (opts.target) return delegatorFn;
};
const fn = () => console.log('!');
on(document.body, 'click', fn); // logs '!' upon clicking the body
on(document.body, 'click', fn, { target: 'p' }); // logs '!' upon clicking a `p` element child of the body
on(document.body, 'click', fn, { options: true }); // use capturing instead of bubbling

onUserInputChangeadvanced

Run the callback whenever the user input type changes (mouse or touch). Useful for enabling/disabling code depending on the input device. This process is dynamic and works with hybrid devices (e.g. touchscreen laptops).

Use two event listeners. Assume mouse input initially and bind a touchstart event listener to the document. On touchstart, add a mousemove event listener to listen for two consecutive mousemove events firing within 20ms, using performance.now(). Run the callback with the input type as an argument in either of these situations.

const onUserInputChange = callback => {
  let type = 'mouse',
    lastTime = 0;
  const mousemoveHandler = () => {
    const now = performance.now();
    if (now - lastTime < 20)
      (type = 'mouse'), callback(type), document.removeEventListener('mousemove', mousemoveHandler);
    lastTime = now;
  };
  document.addEventListener('touchstart', () => {
    if (type === 'touch') return;
    (type = 'touch'), callback(type), document.addEventListener('mousemove', mousemoveHandler);
  });
};
onUserInputChange(type => {
  console.log('The user is now using', type, 'as an input method.');
});

redirect

Redirects to a specified URL.

Use window.location.href or window.location.replace() to redirect to url. Pass a second argument to simulate a link click (true - default) or an HTTP redirect (false).

const redirect = (url, asLink = true) =>
  asLink ? (window.location.href = url) : window.location.replace(url);
redirect('https://google.com');

runAsyncadvanced

Runs a function in a separate thread by using a Web Worker, allowing long running functions to not block the UI.

Create a new Worker using a Blob object URL, the contents of which should be the stringified version of the supplied function. Immediately post the return value of calling the function back. Return a promise, listening for onmessage and onerror events and resolving the data posted back from the worker, or throwing an error.

const runAsync = fn => {
  const blob = `var fn = ${fn.toString()}; postMessage(fn());`;
  const worker = new Worker(
    URL.createObjectURL(new Blob([blob]), {
      type: 'application/javascript; charset=utf-8'
    })
  );
  return new Promise((res, rej) => {
    worker.onmessage = ({ data }) => {
      res(data), worker.terminate();
    };
    worker.onerror = err => {
      rej(err), worker.terminate();
    };
  });
};
const longRunningFunction = () => {
  let result = 0;
  for (let i = 0; i < 1000; i++) {
    for (let j = 0; j < 700; j++) {
      for (let k = 0; k < 300; k++) {
        result = result + i + j + k;
      }
    }
  }
  return result;
};
/*
  NOTE: Since the function is running in a different context, closures are not supported.
  The function supplied to `runAsync` gets stringified, so everything becomes literal.
  All variables and functions must be defined inside.
*/
runAsync(longRunningFunction).then(console.log); // 209685000000
runAsync(() => 10 ** 3).then(console.log); // 1000
let outsideVariable = 50;
runAsync(() => typeof outsideVariable).then(console.log); // 'undefined'

scrollToTop

Smooth-scrolls to the top of the page.

Get distance from top using document.documentElement.scrollTop or document.body.scrollTop. Scroll by a fraction of the distance from the top. Use window.requestAnimationFrame() to animate the scrolling.

const scrollToTop = () => {
  const c = document.documentElement.scrollTop || document.body.scrollTop;
  if (c > 0) {
    window.requestAnimationFrame(scrollToTop);
    window.scrollTo(0, c - c / 8);
  }
};
scrollToTop();

setStyle

Sets the value of a CSS rule for the specified element.

Use element.style to set the value of the CSS rule for the specified element to val.

const setStyle = (el, ruleName, val) => (el.style[ruleName] = val);
setStyle(document.querySelector('p'), 'font-size', '20px'); // The first <p> element on the page will have a font-size of 20px

show

Shows all the elements specified.

Use the spread operator (...) and Array.forEach() to clear the display property for each element specified.

const show = (...el) => [...el].forEach(e => (e.style.display = ''));
show(...document.querySelectorAll('img')); // Shows all <img> elements on the page

toggleClass

Toggle a class for an element.

Use element.classList.toggle() to toggle the specified class for the element.

const toggleClass = (el, className) => el.classList.toggle(className);
toggleClass(document.querySelector('p.special'), 'special'); // The paragraph will not have the 'special' class anymore

UUIDGeneratorBrowser

Generates a UUID in a browser.

Use crypto API to generate a UUID, compliant with RFC4122 version 4.

const UUIDGeneratorBrowser = () =>
  ([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
    (c ^ (crypto.getRandomValues(new Uint8Array(1))[0] & (15 >> (c / 4)))).toString(16)
  );
UUIDGeneratorBrowser(); // '7982fcfe-5721-4632-bede-6000885be57d'

Date

formatDuration

Returns the human readable format of the given number of milliseconds.

Divide ms with the appropriate values to obtain the appropriate values for day, hour, minute, second and millisecond. Use Object.entries() with Array.filter() to keep only non-zero values. Use Array.map() to create the string for each value, pluralizing appropriately. Use String.join(', ') to combine the values into a string.

const formatDuration = ms => {
  if (ms < 0) ms = -ms;
  const time = {
    day: Math.floor(ms / 86400000),
    hour: Math.floor(ms / 3600000) % 24,
    minute: Math.floor(ms / 60000) % 60,
    second: Math.floor(ms / 1000) % 60,
    millisecond: Math.floor(ms) % 1000
  };
  return Object.entries(time)
    .filter(val => val[1] !== 0)
    .map(val => val[1] + ' ' + (val[1] !== 1 ? val[0] + 's' : val[0]))
    .join(', ');
};
formatDuration(1001); // '1 second, 1 millisecond'
formatDuration(34325055574); // '397 days, 6 hours, 44 minutes, 15 seconds, 574 milliseconds'

getDaysDiffBetweenDates

Returns the difference (in days) between two dates.

Calculate the difference (in days) between two Date objects.

const getDaysDiffBetweenDates = (dateInitial, dateFinal) =>
  (dateFinal - dateInitial) / (1000 * 3600 * 24);
getDaysDiffBetweenDates(new Date('2017-12-13'), new Date('2017-12-22')); // 9

tomorrow

Results in a string representation of tomorrow's date. Use new Date() to get today's date, adding 86400000 of seconds to it(24 hours), using Date.toISOString() to convert Date object to string.

const tomorrow = () => new Date(new Date().getTime() + 86400000).toISOString().split('T')[0];
tomorrow(); // 2017-12-27 (if current date is 2017-12-26)

Function

chainAsync

Chains asynchronous functions.

Loop through an array of functions containing asynchronous events, calling next when each asynchronous event has completed.

const chainAsync = fns => {
  let curr = 0;
  const next = () => fns[curr++](next);
  next();
};
chainAsync([
  next => {
    console.log('0 seconds');
    setTimeout(next, 1000);
  },
  next => {
    console.log('1 second');
  }
]);

compose

Performs right-to-left function composition.

Use Array.reduce() to perform right-to-left function composition. The last (rightmost) function can accept one or more arguments; the remaining functions must be unary.

const compose = (...fns) => fns.reduce((f, g) => (...args) => f(g(...args)));
const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = compose(add5, multiply);
multiplyAndAdd5(5, 2); // 15

curry

Curries a function.

Use recursion. If the number of provided arguments (args) is sufficient, call the passed function fn. Otherwise, return a curried function fn that expects the rest of the arguments. If you want to curry a function that accepts a variable number of arguments (a variadic function, e.g. Math.min()), you can optionally pass the number of arguments to the second parameter arity.

const curry = (fn, arity = fn.length, ...args) =>
  arity <= args.length ? fn(...args) : curry.bind(null, fn, arity, ...args);
curry(Math.pow)(2)(10); // 1024
curry(Math.min, 3)(10)(50)(2); // 2

defer

Defers invoking a function until the current call stack has cleared.

Use setTimeout() with a timeout of 1ms to add a new event to the browser event queue and allow the rendering engine to complete its work. Use the spread (...) operator to supply the function with an arbitrary number of arguments.

const defer = (fn, ...args) => setTimeout(fn, 1, ...args);
// Example A:
defer(console.log, 'a'), console.log('b'); // logs 'b' then 'a'

// Example B:
document.querySelector('#someElement').innerHTML = 'Hello';
longRunningFunction(); //Browser will not update the HTML until this has finished
defer(longRunningFunction); // Browser will update the HTML then run the function

functionName

Logs the name of a function.

Use console.debug() and the name property of the passed method to log the method's name to the debug channel of the console.

const functionName = fn => (console.debug(fn.name), fn);
functionName(Math.max); // max (logged in debug channel of console)

memoize

Returns the memoized (cached) function.

Create an empty cache by instantiating a new Map object. Return a function which takes a single argument to be supplied to the memoized function by first checking if the function's output for that specific input value is already cached, or store and return it if not. The function keyword must be used in order to allow the memoized function to have its this context changed if necessary. Allow access to the cache by setting it as a property on the returned function.

const memoize = fn => {
  const cache = new Map();
  const cached = function(val) {
    return cache.has(val) ? cache.get(val) : cache.set(val, fn.call(this, val)) && cache.get(val);
  };
  cached.cache = cache;
  return cached;
};
// See the `anagrams` snippet.
const anagramsCached = memoize(anagrams);
anagramsCached('javascript'); // takes a long time
anagramsCached('javascript'); // returns virtually instantly since it's now cached
console.log(anagramsCached.cache); // The cached anagrams map

negate

Negates a predicate function.

Take a predicate function and apply the not operator (!) to it with its arguments.

const negate = func => (...args) => !func(...args);
[1, 2, 3, 4, 5, 6].filter(negate(n => n % 2 == 0)); // [ 1, 3, 5 ]

once

Ensures a function is called only once.

Utilizing a closure, use a flag, called, and set it to true once the function is called for the first time, preventing it from being called again. In order to allow the function to have its this context changed (such as in an event listener), the function keyword must be used, and the supplied function must have the context applied. Allow the function to be supplied with an arbitrary number of arguments using the rest/spread (...) operator.

const once = fn => {
  let called = false;
  return function(...args) {
    if (called) return;
    called = true;
    return fn.apply(this, args);
  };
};
const startApp = function(event) {
  console.log(this, event); // document.body, MouseEvent
};
document.body.addEventListener('click', once(startApp)); // only runs `startApp` once upon click

runPromisesInSeries

Runs an array of promises in series.

Use Array.reduce() to create a promise chain, where each promise returns the next promise when resolved.

const runPromisesInSeries = ps => ps.reduce((p, next) => p.then(next), Promise.resolve());
const delay = d => new Promise(r => setTimeout(r, d));
runPromisesInSeries([() => delay(1000), () => delay(2000)]); // Executes each promise sequentially, taking a total of 3 seconds to complete

sleep

Delays the execution of an asynchronous function.

Delay executing part of an async function, by putting it to sleep, returning a Promise.

const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
async function sleepyWork() {
  console.log("I'm going to sleep for 1 second.");
  await sleep(1000);
  console.log('I woke up after 1 second.');
}

Math

average

Returns the average of an of two or more numbers.

Use Array.reduce() to add each value to an accumulator, initialized with a value of 0, divide by the length of the array.

const average = (...nums) => [...nums].reduce((acc, val) => acc + val, 0) / nums.length;
average(...[1, 2, 3]); // 2
average(1, 2, 3); // 2

averageBy

Returns the average of an array, after mapping each element to a value using the provided function.

Use Array.map() to map each element to the value returned by fn, Array.reduce() to add each value to an accumulator, initialized with a value of 0, divide by the length of the array.

const averageBy = (arr, fn) =>
  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => acc + val, 0) /
  arr.length;
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 5
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 5

clampNumber

Clamps num within the inclusive range specified by the boundary values a and b.

If num falls within the range, return num. Otherwise, return the nearest number in the range.

const clampNumber = (num, a, b) => Math.max(Math.min(num, Math.max(a, b)), Math.min(a, b));
clampNumber(2, 3, 5); // 3
clampNumber(1, -1, -5); // -1

digitize

Converts a number to an array of digits.

Convert the number to a string, using the spread operator (...) to build an array. Use Array.map() and parseInt() to transform each value to an integer.

const digitize = n => [...`${n}`].map(i => parseInt(i));
digitize(123); // [1, 2, 3]

distance

Returns the distance between two points.

Use Math.hypot() to calculate the Euclidean distance between two points.

const distance = (x0, y0, x1, y1) => Math.hypot(x1 - x0, y1 - y0);
distance(1, 1, 2, 3); // 2.23606797749979

eloadvanced

Computes the new ratings between two or more opponents using the Elo rating system. It takes an array of pre-ratings and returns an array containing post-ratings. The array should be ordered from best performer to worst performer (winner -> loser).

Use the exponent ** operator and math operators to compute the expected score (chance of winning). of each opponent and compute the new rating for each. Loop through the ratings, using each permutation to compute the post-Elo rating for each player in a pairwise fashion. Omit the second argument to use the default kFactor of 32.

const elo = ([...ratings], kFactor = 32, selfRating) => {
  const [a, b] = ratings;
  const expectedScore = (self, opponent) => 1 / (1 + 10 ** ((opponent - self) / 400));
  const newRating = (rating, i) =>
    (selfRating || rating) + kFactor * (i - expectedScore(i ? a : b, i ? b : a));
  if (ratings.length === 2) {
    return [newRating(a, 1), newRating(b, 0)];
  } else {
    for (let i = 0; i < ratings.length; i++) {
      let j = i;
      while (j < ratings.length - 1) {
        [ratings[i], ratings[j + 1]] = elo([ratings[i], ratings[j + 1]], kFactor);
        j++;
      }
    }
  }
  return ratings;
};
// Standard 1v1s
elo([1200, 1200]); // [1216, 1184]
elo([1200, 1200], 64); // [1232, 1168]
// 4 player FFA, all same rank
elo([1200, 1200, 1200, 1200]).map(Math.round); // [1246, 1215, 1185, 1154]
/*
For teams, each rating can adjusted based on own team's average rating vs.
average rating of opposing team, with the score being added to their
own individual rating by supplying it as the third argument.
*/

factorial

Calculates the factorial of a number.

Use recursion. If n is less than or equal to 1, return 1. Otherwise, return the product of n and the factorial of n - 1. Throws an exception if n is a negative number.

const factorial = n =>
  n < 0
    ? (() => {
        throw new TypeError('Negative numbers are not allowed!');
      })()
    : n <= 1 ? 1 : n * factorial(n - 1);
factorial(6); // 720

fibonacci

Generates an array, containing the Fibonacci sequence, up until the nth term.

Create an empty array of the specific length, initializing the first two values (0 and 1). Use Array.reduce() to add values into the array, using the sum of the last two values, except for the first two.

const fibonacci = n =>
  Array.from({ length: n }).reduce(
    (acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i),
    []
  );
fibonacci(6); // [0, 1, 1, 2, 3, 5]

gcd

Calculates the greatest common divisor between two or more numbers/arrays.

The inner _gcd function uses recursion. Base case is when y equals 0. In this case, return x. Otherwise, return the GCD of y and the remainder of the division x/y.

const gcd = (...arr) => {
  const _gcd = (x, y) => (!y ? x : gcd(y, x % y));
  return [...arr].reduce((a, b) => _gcd(a, b));
};
gcd(8, 36); // 4
gcd(...[12, 8, 32]); // 4

geometricProgression

Initializes an array containing the numbers in the specified range where start and end are inclusive and the ratio between two terms is step. Returns an error if step equals 1.

Use Array.from(), Math.log() and Math.floor() to create an array of the desired length, Array.map() to fill with the desired values in a range. Omit the second argument, start, to use a default value of 1. Omit the third argument, step, to use a default value of 2.

const geometricProgression = (end, start = 1, step = 2) =>
  Array.from({ length: Math.floor(Math.log(end / start) / Math.log(step)) + 1 }).map(
    (v, i) => start * step ** i
  );
geometricProgression(256); // [1, 2, 4, 8, 16, 32, 64, 128, 256]
geometricProgression(256, 3); // [3, 6, 12, 24, 48, 96, 192]
geometricProgression(256, 1, 4); // [1, 4, 16, 64, 256]

hammingDistance

Calculates the Hamming distance between two values.

Use XOR operator (^) to find the bit difference between the two numbers, convert to a binary string using toString(2). Count and return the number of 1s in the string, using match(/1/g).

const hammingDistance = (num1, num2) => ((num1 ^ num2).toString(2).match(/1/g) || '').length;
hammingDistance(2, 3); // 1

inRange

Checks if the given number falls within the given range.

Use arithmetic comparison to check if the given number is in the specified range. If the second parameter, end, is not specified, the range is considered to be from 0 to start.

const inRange = (n, start, end = null) => {
  if (end && start > end) end = [start, (start = end)][0];
  return end == null ? n >= 0 && n < start : n >= start && n < end;
};
inRange(3, 2, 5); // true
inRange(3, 4); // true
inRange(2, 3, 5); // false
inrange(3, 2); // false

isDivisible

Checks if the first numeric argument is divisible by the second one.

Use the modulo operator (%) to check if the remainder is equal to 0.

const isDivisible = (dividend, divisor) => dividend % divisor === 0;
isDivisible(6, 3); // true

isEven

Returns true if the given number is even, false otherwise.

Checks whether a number is odd or even using the modulo (%) operator. Returns true if the number is even, false if the number is odd.

const isEven = num => num % 2 === 0;
isEven(3); // false

isPrime

Checks if the provided integer is a prime number.

Check numbers from 2 to the square root of the given number. Return false if any of them divides the given number, else return true, unless the number is less than 2.

const isPrime = num => {
  const boundary = Math.floor(Math.sqrt(num));
  for (var i = 2; i <= boundary; i++) if (num % i == 0) return false;
  return num >= 2;
};
isPrime(11); // true

lcm

Returns the least common multiple of two or more numbers.

Use the greatest common divisor (GCD) formula and the fact that lcm(x,y) = x * y / gcd(x,y) to determine the least common multiple. The GCD formula uses recursion.

const lcm = (...arr) => {
  const gcd = (x, y) => (!y ? x : gcd(y, x % y));
  const _lcm = (x, y) => x * y / gcd(x, y);
  return [...arr].reduce((a, b) => _lcm(a, b));
};
lcm(12, 7); // 84
lcm(...[1, 3, 4, 5]); // 60

luhnCheck

Implementation of the Luhn Algorithm used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers etc.

Use String.split(''), Array.reverse() and Array.map() in combination with parseInt() to obtain an array of digits. Use Array.splice(0,1) to obtain the last digit. Use Array.reduce() to implement the Luhn Algorithm. Return true if sum is divisible by 10, false otherwise.

const luhnCheck = num => {
  let arr = (num + '')
    .split('')
    .reverse()
    .map(x => parseInt(x));
  let lastDigit = arr.splice(0, 1)[0];
  let sum = arr.reduce((acc, val, i) => (i % 2 !== 0 ? acc + val : acc + (val * 2) % 9 || 9), 0);
  sum += lastDigit;
  return sum % 10 === 0;
};
luhnCheck('4485275742308327'); // true
luhnCheck(6011329933655299); //  false
luhnCheck(123456789); // false

maxBy

Returns the maximum value of an array, after mapping each element to a value using the provided function.

Use Array.map() to map each element to the value returned by fn, Math.max() to get the maximum value.

const maxBy = (arr, fn) => Math.max(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 8
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 8

median

Returns the median of an array of numbers.

Find the middle of the array, use Array.sort() to sort the values. Return the number at the midpoint if length is odd, otherwise the average of the two middle numbers.

const median = arr => {
  const mid = Math.floor(arr.length / 2),
    nums = [...arr].sort((a, b) => a - b);
  return arr.length % 2 !== 0 ? nums[mid] : (nums[mid - 1] + nums[mid]) / 2;
};
median([5, 6, 50, 1, -5]); // 5

minBy

Returns the minimum value of an array, after mapping each element to a value using the provided function.

Use Array.map() to map each element to the value returned by fn, Math.min() to get the maximum value.

const minBy = (arr, fn) => Math.min(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 8
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 8

percentile

Uses the percentile formula to calculate how many numbers in the given array are less or equal to the given value.

Use Array.reduce() to calculate how many numbers are below the value and how many are the same value and apply the percentile formula.

const percentile = (arr, val) =>
  100 * arr.reduce((acc, v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0), 0) / arr.length;
percentile([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 6); // 55

powerset

Returns the powerset of a given array of numbers.

Use Array.reduce() combined with Array.map() to iterate over elements and combine into an array containing all combinations.

const powerset = arr => arr.reduce((a, v) => a.concat(a.map(r => [v].concat(r))), [[]]);
powerset([1, 2]); // [[], [1], [2], [2,1]]

primes

Generates primes up to a given number, using the Sieve of Eratosthenes.

Generate an array from 2 to the given number. Use Array.filter() to filter out the values divisible by any number from 2 to the square root of the provided number.

const primes = num => {
  let arr = Array.from({ length: num - 1 }).map((x, i) => i + 2),
    sqroot = Math.floor(Math.sqrt(num)),
    numsTillSqroot = Array.from({ length: sqroot - 1 }).map((x, i) => i + 2);
  numsTillSqroot.forEach(x => (arr = arr.filter(y => y % x !== 0 || y == x)));
  return arr;
};
primes(10); // [2,3,5,7]

randomIntArrayInRange

Returns an array of n random integers in the specified range.

Use Array.from() to create an empty array of the specific length, Math.random() to generate a random number and map it to the desired range, using Math.floor() to make it an integer.

const randomIntArrayInRange = (min, max, n = 1) =>
  Array.from({ length: n }, () => Math.floor(Math.random() * (max - min + 1)) + min);
randomIntArrayInRange(12, 35, 10); // [ 34, 14, 27, 17, 30, 27, 20, 26, 21, 14 ]

randomIntegerInRange

Returns a random integer in the specified range.

Use Math.random() to generate a random number and map it to the desired range, using Math.floor() to make it an integer.

const randomIntegerInRange = (min, max) => Math.floor(Math.random() * (max - min + 1)) + min;
randomIntegerInRange(0, 5); // 2

randomNumberInRange

Returns a random number in the specified range.

Use Math.random() to generate a random value, map it to the desired range using multiplication.

const randomNumberInRange = (min, max) => Math.random() * (max - min) + min;
randomNumberInRange(2, 10); // 6.0211363285087005

round

Rounds a number to a specified amount of digits.

Use Math.round() and template literals to round the number to the specified number of digits. Omit the second argument, decimals to round to an integer.

const round = (n, decimals = 0) => Number(`${Math.round(`${n}e${decimals}`)}e-${decimals}`);
round(1.005, 2); // 1.01

sdbm

Hashes the input string into a whole number.

Use String.split('') and Array.reduce() to create a hash of the input string, utilizing bit shifting.

const sdbm = str => {
  let arr = str.split('');
  return arr.reduce(
    (hashCode, currentVal) =>
      (hashCode = currentVal.charCodeAt(0) + (hashCode << 6) + (hashCode << 16) - hashCode),
    0
  );
};
sdbm('name'); // -3521204949

standardDeviation

Returns the standard deviation of an array of numbers.

Use Array.reduce() to calculate the mean, variance and the sum of the variance of the values, the variance of the values, then determine the standard deviation. You can omit the second argument to get the sample standard deviation or set it to true to get the population standard deviation.

const standardDeviation = (arr, usePopulation = false) => {
  const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
  return Math.sqrt(
    arr.reduce((acc, val) => acc.concat((val - mean) ** 2), []).reduce((acc, val) => acc + val, 0) /
      (arr.length - (usePopulation ? 0 : 1))
  );
};
standardDeviation([10, 2, 38, 23, 38, 23, 21]); // 13.284434142114991 (sample)
standardDeviation([10, 2, 38, 23, 38, 23, 21], true); // 12.29899614287479 (population)

sum

Returns the sum of two or more numbers/arrays.

Use Array.reduce() to add each value to an accumulator, initialized with a value of 0.

const sum = (...arr) => [...arr].reduce((acc, val) => acc + val, 0);
sum(...[1, 2, 3, 4]); // 10

sumBy

Returns the sum of an array, after mapping each element to a value using the provided function.

Use Array.map() to map each element to the value returned by fn, Array.reduce() to add each value to an accumulator, initialized with a value of 0.

const sumBy = (arr, fn) =>
  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => acc + val, 0);
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 20
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20

sumPower

Returns the sum of the powers of all the numbers from start to end (both inclusive).

Use Array.fill() to create an array of all the numbers in the target range, Array.map() and the exponent operator (**) to raise them to power and Array.reduce() to add them together. Omit the second argument, power, to use a default power of 2. Omit the third argument, start, to use a default starting value of 1.

const sumPower = (end, power = 2, start = 1) =>
  Array(end + 1 - start)
    .fill(0)
    .map((x, i) => (i + start) ** power)
    .reduce((a, b) => a + b, 0);
sumPower(10); // 385
sumPower(10, 3); //3025
sumPower(10, 3, 5); //2925

toSafeInteger

Converts a value to a safe integer.

Use Math.max() and Math.min() to find the closest safe value. Use Math.round() to convert to an integer.

const toSafeInteger = num =>
  Math.round(Math.max(Math.min(num, Number.MAX_SAFE_INTEGER), Number.MIN_SAFE_INTEGER));
toSafeInteger('3.2'); // 3
toSafeInteger(Infinity); // 9007199254740991

Node

atob

Decodes a string of data which has been encoded using base-64 encoding.

Create a Buffer for the given string with base-64 encoding and use Buffer.toString('binary') to return the decoded string.

const atob = str => new Buffer(str, 'base64').toString('binary');
atob('Zm9vYmFy'); // 'foobar'

btoa

Creates a base-64 encoded ASCII string from a String object in which each character in the string is treated as a byte of binary data.

Create a Buffer for the given string with binary encoding and use Buffer.toString('base64') to return the encoded string.

const btoa = str => new Buffer(str, 'binary').toString('base64');
btoa('foobar'); // 'Zm9vYmFy'

colorize

Add special characters to text to print in color in the console (combined with console.log()).

Use template literals and special characters to add the appropriate color code to the string output. For background colors, add a special character that resets the background color at the end of the string.

const colorize = (...args) => ({
  black: `\x1b[30m${args.join(' ')}`,
  red: `\x1b[31m${args.join(' ')}`,
  green: `\x1b[32m${args.join(' ')}`,
  yellow: `\x1b[33m${args.join(' ')}`,
  blue: `\x1b[34m${args.join(' ')}`,
  magenta: `\x1b[35m${args.join(' ')}`,
  cyan: `\x1b[36m${args.join(' ')}`,
  white: `\x1b[37m${args.join(' ')}`,
  bgBlack: `\x1b[40m${args.join(' ')}\x1b[0m`,
  bgRed: `\x1b[41m${args.join(' ')}\x1b[0m`,
  bgGreen: `\x1b[42m${args.join(' ')}\x1b[0m`,
  bgYellow: `\x1b[43m${args.join(' ')}\x1b[0m`,
  bgBlue: `\x1b[44m${args.join(' ')}\x1b[0m`,
  bgMagenta: `\x1b[45m${args.join(' ')}\x1b[0m`,
  bgCyan: `\x1b[46m${args.join(' ')}\x1b[0m`,
  bgWhite: `\x1b[47m${args.join(' ')}\x1b[0m`
});
console.log(colorize('foo').red); // 'foo' (red letters)
console.log(colorize('foo', 'bar').bgBlue); // 'foo bar' (blue background)
console.log(colorize(colorize('foo').yellow, colorize('foo').green).bgWhite); // 'foo bar' (first word in yellow letters, second word in green letters, white background for both)

hasFlags

Check if the current process's arguments contain the specified flags.

Use Array.every() and Array.includes() to check if process.argv contains all the specified flags. Use a regular expression to test if the specified flags are prefixed with - or -- and prefix them accordingly.

const hasFlags = (...flags) =>
  flags.every(flag => process.argv.includes(/^-{1,2}/.test(flag) ? flag : '--' + flag));
// node myScript.js -s --test --cool=true
hasFlags('-s'); // true
hasFlags('--test', 'cool=true', '-s'); // true
hasFlags('special'); // false

hashNode

Creates a hash for a value using the SHA-256 algorithm. Returns a promise.

Use crypto API to create a hash for the given value.

const crypto = require('crypto');
const hashNode = val =>
  new Promise(resolve =>
    setTimeout(
      () =>
        resolve(
          crypto
            .createHash('sha256')
            .update(val)
            .digest('hex')
        ),
      0
    )
  );
hashBrowser(JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })).then(console.log); // '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'

isTravisCI

Checks if the current environment is Travis CI.

Checks if the current environment has the TRAVIS and CI environment variables (reference).

const isTravisCI = () => 'TRAVIS' in process.env && 'CI' in process.env;
isTravisCI(); // true (if code is running on Travis CI)

JSONToFile

Writes a JSON object to a file.

Use fs.writeFile(), template literals and JSON.stringify() to write a json object to a .json file.

const fs = require('fs');
const JSONToFile = (obj, filename) =>
  fs.writeFile(`${filename}.json`, JSON.stringify(obj, null, 2));
JSONToFile({ test: 'is passed' }, 'testJsonFile'); // writes the object to 'testJsonFile.json'

readFileLines

Returns an array of lines from the specified file.

Use readFileSync function in fs node package to create a Buffer from a file. convert buffer to string using toString(encoding) function. creating an array from contents of file by spliting file content line by line (each \n).

const fs = require('fs');
const readFileLines = filename =>
  fs
    .readFileSync(filename)
    .toString('UTF8')
    .split('\n');
/*
contents of test.txt :
  line1
  line2
  line3
  ___________________________
*/
let arr = readFileLines('test.txt');
console.log(arr); // ['line1', 'line2', 'line3']

untildify

Converts a tilde path to an absolute path.

Use String.replace() with a regular expression and OS.homedir() to replace the ~ in the start of the path with the home directory.

const untildify = str => str.replace(/^~($|\/|\\)/, `${require('os').homedir()}$1`);
untildify('~/node'); // '/Users/aUser/node'

UUIDGeneratorNode

Generates a UUID in Node.JS.

Use crypto API to generate a UUID, compliant with RFC4122 version 4.

const crypto = require('crypto');
const UUIDGeneratorNode = () =>
  ([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
    (c ^ (crypto.randomBytes(1)[0] & (15 >> (c / 4)))).toString(16)
  );
UUIDGeneratorNode(); // '79c7c136-60ee-40a2-beb2-856f1feabefc'

Object

defaults

Assigns default values for all properties in an object that are undefined.

Use Object.assign() to create a new empty object and copy the original one to maintain key order, use Array.reverse() and the spread operator ... to combine the default values from left to right, finally use obj again to overwrite properties that originally had a value.

const defaults = (obj, ...defs) => Object.assign({}, obj, ...defs.reverse(), obj);
defaults({ a: 1 }, { b: 2 }, { b: 6 }, { a: 3 }); // { a: 1, b: 2 }

equalsadvanced

Performs a deep comparison between two values to determine if they are equivalent.

Check if the two values are identical, if they are both Date objects with the same time, using Date.getTime() or if they are both non-object values with an equivalent value (strict comparison). Check if only one value is null or undefined or if their prototypes differ. If none of the above conditions are met, use Object.keys() to check if both values have the same number of keys, then use Array.every() to check if every key in the first value exists in the second one and if they are equivalent by calling this method recursively.

const equals = (a, b) => {
  if (a === b) return true;
  if (a instanceof Date && b instanceof Date) return a.getTime() === b.getTime();
  if (!a || !b || (typeof a != 'object' && typeof b !== 'object')) return a === b;
  if (a === null || a === undefined || b === null || b === undefined) return false;
  if (a.prototype !== b.prototype) return false;
  let keys = Object.keys(a);
  if (keys.length !== Object.keys(b).length) return false;
  return keys.every(k => equals(a[k], b[k]));
};
equals({ a: [2, { e: 3 }], b: [4], c: 'foo' }, { a: [2, { e: 3 }], b: [4], c: 'foo' }); // true

forOwn

Iterates over all own properties of an object, running a callback for each one.

Use Object.keys(obj) to get all the properties of the object, Array.forEach() to run the provided function for each key-value pair. The callback receives three arguments - the value, the key and the object.

const forOwn = (obj, fn) => Object.keys(obj).forEach(key => fn(obj[key], key, obj));
forOwn({ foo: 'bar', a: 1 }, v => console.log(v)); // 'bar', 1

forOwnRight

Iterates over all own properties of an object in reverse, running a callback for each one.

Use Object.keys(obj) to get all the properties of the object, Array.reverse() to reverse their order and Array.forEach() to run the provided function for each key-value pair. The callback receives three arguments - the value, the key and the object.

const forOwnRight = (obj, fn) =>
  Object.keys(obj)
    .reverse()
    .forEach(key => fn(obj[key], key, obj));
forOwnRight({ foo: 'bar', a: 1 }, v => console.log(v)); // 1, 'bar'

functions

Returns an array of function property names from own (and optionally inherited) enumerable properties of an object.

Use Object.keys(obj) to iterate over the object's own properties. If inherited is true, use Object.get.PrototypeOf(obj) to also get the object's inherited properties. Use Array.filter() to keep only those properties that are functions. Omit the second argument, inherited, to not include inherited properties by default.

const functions = (obj, inherited = false) =>
  (inherited
    ? [...Object.keys(obj), ...Object.keys(Object.getPrototypeOf(obj))]
    : Object.keys(obj)
  ).filter(key => typeof obj[key] === 'function');
function Foo() {
  this.a = () => 1;
  this.b = () => 2;
}
Foo.prototype.c = () => 3;
functions(new Foo()); // ['a', 'b']
functions(new Foo(), true); // ['a', 'b', 'c']

get

Retrieve a set of properties indicated by the given selectors from an object.

Use Array.map() for each selector, String.replace() to replace square brackets with dots, String.split('.') to split each selector, Array.filter() to remove empty values and Array.reduce() to get the value indicated by it.

const get = (from, ...selectors) =>
  [...selectors].map(s =>
    s
      .replace(/\[([^\[\]]*)\]/g, '.$1.')
      .split('.')
      .filter(t => t !== '')
      .reduce((prev, cur) => prev && prev[cur], from)
  );
const obj = { selector: { to: { val: 'val to select' } }, target: [1, 2, { a: 'test' }] };
get(obj, 'selector.to.val', 'target[0]', 'target[2].a'); // ['val to select', 1, 'test']

invertKeyValues

Inverts the key-value pairs of an object, without mutating it.

Use Object.keys() and Array.reduce() to invert the key-value pairs of an object.

const invertKeyValues = obj =>
  Object.keys(obj).reduce((acc, key) => {
    acc[obj[key]] = key;
    return acc;
  }, {});
invertKeyValues({ name: 'John', age: 20 }); // { 20: 'age', John: 'name' }

lowercaseKeys

Creates a new object from the specified object, where all the keys are in lowercase.

Use Object.keys() and Array.reduce() to create a new object from the specified object. Convert each key in the original object to lowercase, using String.toLowerCase().

const lowercaseKeys = obj =>
  Object.keys(obj).reduce((acc, key) => {
    acc[key.toLowerCase()] = obj[key];
    return acc;
  }, {});
const myObj = { Name: 'Adam', sUrnAME: 'Smith' };
const myObjLower = lowercaseKeys(myObj); // {name: 'Adam', surname: 'Smith'};

mapKeys

Creates an object with keys generated by running the provided function for each key and the same values as the provided object.

Use Object.keys(obj) to iterate over the object's keys. Use Array.reduce() to create a new object with the same values and mapped keys using fn.

const mapKeys = (obj, fn) =>
  Object.keys(obj).reduce((acc, k) => {
    acc[fn(obj[k], k, obj)] = obj[k];
    return acc;
  }, {});
mapKeys({ a: 1, b: 2 }, (val, key) => key + val); // { a1: 1, b2: 2 }

mapValues

Creates an object with the same keys as the provided object and values generated by running the provided function for each value.

Use Object.keys(obj) to iterate over the object's keys. Use Array.reduce() to create a new object with the same keys and mapped values using fn.

const mapValues = (obj, fn) =>
  Object.keys(obj).reduce((acc, k) => {
    acc[k] = fn(obj[k], k, obj);
    return acc;
  }, {});
const users = {
  fred: { user: 'fred', age: 40 },
  pebbles: { user: 'pebbles', age: 1 }
};
mapValues(users, u => u.age); // { fred: 40, pebbles: 1 }

merge

Creates a new object from the combination of two or more objects.

Use Array.reduce() combined with Object.keys(obj) to iterate over all objects and keys. Use hasOwnProperty() and Array.concat() to append values for keys existing in multiple objects.

const merge = (...objs) =>
  [...objs].reduce(
    (acc, obj) =>
      Object.keys(obj).reduce((a, k) => {
        acc[k] = acc.hasOwnProperty(k) ? [].concat(acc[k]).concat(obj[k]) : obj[k];
        return acc;
      }, {}),
    {}
  );
const object = {
  a: [{ x: 2 }, { y: 4 }],
  b: 1
};
const other = {
  a: { z: 3 },
  b: [2, 3],
  c: 'foo'
};
merge(object, other); // { a: [ { x: 2 }, { y: 4 }, { z: 3 } ], b: [ 1, 2, 3 ], c: 'foo' }

objectFromPairs

Creates an object from the given key-value pairs.

Use Array.reduce() to create and combine key-value pairs.

const objectFromPairs = arr => arr.reduce((a, v) => ((a[v[0]] = v[1]), a), {});
objectFromPairs([['a', 1], ['b', 2]]); // {a: 1, b: 2}

objectToPairs

Creates an array of key-value pair arrays from an object.

Use Object.keys() and Array.map() to iterate over the object's keys and produce an array with key-value pairs.

const objectToPairs = obj => Object.keys(obj).map(k => [k, obj[k]]);
objectToPairs({ a: 1, b: 2 }); // [['a',1],['b',2]]

omit

Omits the key-value pairs corresponding to the given keys from an object.

Use Object.keys(obj), Array.filter() and Array.includes() to remove the provided keys. Use Array.reduce() to convert the filtered keys back to an object with the corresponding key-value pairs.

const omit = (obj, arr) =>
  Object.keys(obj)
    .filter(k => !arr.includes(k))
    .reduce((acc, key) => ((acc[key] = obj[key]), acc), {});
omit({ a: 1, b: '2', c: 3 }, ['b']); // { 'a': 1, 'c': 3 }

omitBy

Creates an object composed of the properties the given function returns falsey for. The function is invoked with two arguments: (value, key).

Use Object.keys(obj) and Array.filter()to remove the keys for which fn returns a truthy value. Use Array.reduce() to convert the filtered keys back to an object with the corresponding key-value pairs.

const omitBy = (obj, fn) =>
  Object.keys(obj)
    .filter(k => !fn(obj[k], k))
    .reduce((acc, key) => ((acc[key] = obj[key]), acc), {});
omitBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number'); // { b: '2' }

orderBy

Returns a sorted array of objects ordered by properties and orders.

Uses Array.sort(), Array.reduce() on the props array with a default value of 0, use array destructuring to swap the properties position depending on the order passed. If no orders array is passed it sort by 'asc' by default.

const orderBy = (arr, props, orders) =>
  [...arr].sort((a, b) =>
    props.reduce((acc, prop, i) => {
      if (acc === 0) {
        const [p1, p2] = orders && orders[i] === 'desc' ? [b[prop], a[prop]] : [a[prop], b[prop]];
        acc = p1 > p2 ? 1 : p1 < p2 ? -1 : 0;
      }
      return acc;
    }, 0)
  );
const users = [{ name: 'fred', age: 48 }, { name: 'barney', age: 36 }, { name: 'fred', age: 40 }];
orderBy(users, ['name', 'age'], ['asc', 'desc']); // [{name: 'barney', age: 36}, {name: 'fred', age: 48}, {name: 'fred', age: 40}]
orderBy(users, ['name', 'age']); // [{name: 'barney', age: 36}, {name: 'fred', age: 40}, {name: 'fred', age: 48}]

pick

Picks the key-value pairs corresponding to the given keys from an object.

Use Array.reduce() to convert the filtered/picked keys back to an object with the corresponding key-value pairs if the key exists in the object.

const pick = (obj, arr) =>
  arr.reduce((acc, curr) => (curr in obj && (acc[curr] = obj[curr]), acc), {});
pick({ a: 1, b: '2', c: 3 }, ['a', 'c']); // { 'a': 1, 'c': 3 }

pickBy

Creates an object composed of the properties the given function returns truthy for. The function is invoked with two arguments: (value, key).

Use Object.keys(obj) and Array.filter()to remove the keys for which fn returns a falsey value. Use Array.reduce() to convert the filtered keys back to an object with the corresponding key-value pairs.

const pickBy = (obj, fn) =>
  Object.keys(obj)
    .filter(k => fn(obj[k], k))
    .reduce((acc, key) => ((acc[key] = obj[key]), acc), {});
pickBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number'); // { 'a': 1, 'c': 3 }

shallowClone

Creates a shallow clone of an object.

Use Object.assign() and an empty object ({}) to create a shallow clone of the original.

const shallowClone = obj => Object.assign({}, obj);
const a = { x: true, y: 1 };
const b = shallowClone(a); // a !== b

size

Get size of arrays, objects or strings.

Get type of val (array, object or string). Use length property for arrays. Use length or size value if available or number of keys for objects. Use size of a Blob object created from val for strings.

Split strings into array of characters with split('') and return its length.

const size = val =>
  Array.isArray(val)
    ? val.length
    : val && typeof val === 'object'
      ? val.size || val.length || Object.keys(val).length
      : typeof val === 'string' ? new Blob([val]).size : 0;
size([1, 2, 3, 4, 5]); // 5
size('size'); // 4
size({ one: 1, two: 2, three: 3 }); // 3

transform

Applies a function against an accumulator and each key in the object (from left to right).

Use Object.keys(obj) to iterate over each key in the object, Array.reduce() to call the apply the specified function against the given accumulator.

const transform = (obj, fn, acc) => Object.keys(obj).reduce((a, k) => fn(a, obj[k], k, obj), acc);
transform(
  { a: 1, b: 2, c: 1 },
  (r, v, k) => {
    (r[v] || (r[v] = [])).push(k);
    return r;
  },
  {}
); // { '1': ['a', 'c'], '2': ['b'] }

truthCheckCollection

Checks if the predicate (second argument) is truthy on all elements of a collection (first argument).

Use Array.every() to check if each passed object has the specified property and if it returns a truthy value.

const truthCheckCollection = (collection, pre) => collection.every(obj => obj[pre]);
truthCheckCollection([{ user: 'Tinky-Winky', sex: 'male' }, { user: 'Dipsy', sex: 'male' }], 'sex'); // true

String

anagrams

⚠️ WARNING: This function's execution time increases exponentially with each character. Anything more than 8 to 10 characters will cause your browser to hang as it tries to solve all the different combinations.

Generates all anagrams of a string (contains duplicates).

Use recursion. For each letter in the given string, create all the partial anagrams for the rest of its letters. Use Array.map() to combine the letter with each partial anagram, then Array.reduce() to combine all anagrams in one array. Base cases are for string length equal to 2 or 1.

const anagrams = str => {
  if (str.length <= 2) return str.length === 2 ? [str, str[1] + str[0]] : [str];
  return str
    .split('')
    .reduce(
      (acc, letter, i) =>
        acc.concat(anagrams(str.slice(0, i) + str.slice(i + 1)).map(val => letter + val)),
      []
    );
};
anagrams('abc'); // ['abc','acb','bac','bca','cab','cba']

byteSize

Returns the length of a string in bytes.

Convert a given string to a Blob Object and find its size.

const byteSize = str => new Blob([str]).size;
byteSize('😀'); // 4
byteSize('Hello World'); // 11

capitalize

Capitalizes the first letter of a string.

Use array destructuring and String.toUpperCase() to capitalize first letter, ...rest to get array of characters after first letter and then Array.join('') to make it a string again. Omit the lowerRest parameter to keep the rest of the string intact, or set it to true to convert to lowercase.

const capitalize = ([first, ...rest], lowerRest = false) =>
  first.toUpperCase() + (lowerRest ? rest.join('').toLowerCase() : rest.join(''));
capitalize('fooBar'); // 'FooBar'
capitalize('fooBar', true); // 'Foobar'

capitalizeEveryWord

Capitalizes the first letter of every word in a string.

Use String.replace() to match the first character of each word and String.toUpperCase() to capitalize it.

const capitalizeEveryWord = str => str.replace(/\b[a-z]/g, char => char.toUpperCase());
capitalizeEveryWord('hello world!'); // 'Hello World!'

decapitalize

Decapitalizes the first letter of a string.

Use array destructuring and String.toLowerCase() to decapitalize first letter, ...rest to get array of characters after first letter and then Array.join('') to make it a string again. Omit the upperRest parameter to keep the rest of the string intact, or set it to true to convert to uppercase.

const decapitalize = ([first, ...rest], upperRest = false) =>
  first.toLowerCase() + (upperRest ? rest.join('').toUpperCase() : rest.join(''));
decapitalize('FooBar'); // 'fooBar'
decapitalize('FooBar', true); // 'fOOBAR'

escapeHTML

Escapes a string for use in HTML.

Use String.replace() with a regexp that matches the characters that need to be escaped, using a callback function to replace each character instance with its associated escaped character using a dictionary (object).

const escapeHTML = str =>
  str.replace(
    /[&<>'"]/g,
    tag =>
      ({
        '&': '&amp;',
        '<': '&lt;',
        '>': '&gt;',
        "'": '&#39;',
        '"': '&quot;'
      }[tag] || tag)
  );
escapeHTML('<a href="#">Me & you</a>'); // '&lt;a href=&quot;#&quot;&gt;Me &amp; you&lt;/a&gt;'

escapeRegExp

Escapes a string to use in a regular expression.

Use String.replace() to escape special characters.

const escapeRegExp = str => str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
escapeRegExp('(test)'); // \\(test\\)

fromCamelCase

Converts a string from camelcase.

Use String.replace() to remove underscores, hyphens, and spaces and convert words to camelcase. Omit the second argument to use a default separator of _.

const fromCamelCase = (str, separator = '_') =>
  str
    .replace(/([a-z\d])([A-Z])/g, '$1' + separator + '$2')
    .replace(/([A-Z]+)([A-Z][a-z\d]+)/g, '$1' + separator + '$2')
    .toLowerCase();
fromCamelCase('someDatabaseFieldName', ' '); // 'some database field name'
fromCamelCase('someLabelThatNeedsToBeCamelized', '-'); // 'some-label-that-needs-to-be-camelized'
fromCamelCase('someJavascriptProperty', '_'); // 'some_javascript_property'

isAbsoluteURL

Returns true if the given string is an absolute URL, false otherwise.

Use a regular expression to test if the string is an absolute URL.

const isAbsoluteURL = str => /^[a-z][a-z0-9+.-]*:/.test(str);
isAbsoluteURL('https://google.com'); // true
isAbsoluteURL('ftp://www.myserver.net'); // true
isAbsoluteURL('/foo/bar'); // false

isLowerCase

Checks if a string is lower case.

Convert the given string to lower case, using String.toLowerCase() and compare it to the original.

const isLowerCase = str => str === str.toLowerCase();
isLowerCase('abc'); // true
isLowerCase('[email protected]$'); // true
isLowerCase('Ab4'); // false

isUpperCase

Checks if a string is upper case.

Convert the given string to upper case, using String.toUpperCase() and compare it to the original.

const isUpperCase = str => str === str.toUpperCase();
isUpperCase('ABC'); // true
isLowerCase('[email protected]$'); // true
isLowerCase('aB4'); // false

mask

Replaces all but the last num of characters with the specified mask character.

Use String.slice() to grab the portion of the characters that need to be masked and use String.replace() with a regexp to replace every character with the mask character. Concatenate the masked characters with the remaining unmasked portion of the string. Omit the second argument, num, to keep a default of 4 characters unmasked. If num is negative, the unmasked characters will be at the start of the string. Omit the third argument, mask, to use a default character of '*' for the mask.

const mask = (cc, num = 4, mask = '*') =>
  ('' + cc).slice(0, -num).replace(/./g, mask) + ('' + cc).slice(-num);
mask(1234567890); // '******7890'
mask(1234567890, 3); // '*******890'
mask(1234567890, -4, '$'); // '$$$$567890'

palindrome

Returns true if the given string is a palindrome, false otherwise.

Convert string String.toLowerCase() and use String.replace() to remove non-alphanumeric characters from it. Then, String.split('') into individual characters, Array.reverse(), String.join('') and compare to the original, unreversed string, after converting it String.tolowerCase().

const palindrome = str => {
  const s = str.toLowerCase().replace(/[\W_]/g, '');
  return (
    s ===
    s
      .split('')
      .reverse()
      .join('')
  );
};
palindrome('taco cat'); // true

pluralize

Returns the singular or plural form of the word based on the input number. If the first argument is an object, it will use a closure by returning a function that can auto-pluralize words that don't simply end in s if the supplied dictionary contains the word.

If num is either -1 or 1, return the singular form of the word. If num is any other number, return the plural form. Omit the third argument to use the default of the singular word + s, or supply a custom pluralized word when necessary. If the first argument is an object, utilize a closure by returning a function which can use the supplied dictionary to resolve the correct plural form of the word.

const pluralize = (val, word, plural = word + 's') => {
  const _pluralize = (num, word, plural = word + 's') =>
    [1, -1].includes(Number(num)) ? word : plural;
  if (typeof val === 'object') return (num, word) => _pluralize(num, word, val[word]);
  return _pluralize(val, word, plural);
};
pluralize(0, 'apple'); // 'apples'
pluralize(1, 'apple'); // 'apple'
pluralize(2, 'apple'); // 'apples'
pluralize(2, 'person', 'people'); // 'people'

const PLURALS = {
  person: 'people',
  radius: 'radii'
};
const autoPluralize = pluralize(PLURALS);
autoPluralize(2, 'person'); // 'people'

reverseString

Reverses a string.

Use the spread operator (...) and Array.reverse() to reverse the order of the characters in the string. Combine characters to get a string using String.join('').

const reverseString = str => [...str].reverse().join('');
reverseString('foobar'); // 'raboof'

sortCharactersInString

Alphabetically sorts the characters in a string.

Use the spread operator (...), Array.sort() and String.localeCompare() to sort the characters in str, recombine using String.join('').

const sortCharactersInString = str => [...str].sort((a, b) => a.localeCompare(b)).join('');
sortCharactersInString('cabbage'); // 'aabbceg'

splitLines

Splits a multiline string into an array of lines.

Use String.split() and a regular expression to match line breaks and create an array.

const splitLines = str => str.split(/\r?\n/);
splitLines('This\nis a\nmultiline\nstring.\n'); // ['This', 'is a', 'multiline', 'string.' , '']

toCamelCase

Converts a string to camelcase.

Break the string into words and combine them capitalizing the first letter of each word, using a regexp.

const toCamelCase = str => {
  let s =
    str &&
    str
      .match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
      .map(x => x.slice(0, 1).toUpperCase() + x.slice(1).toLowerCase())
      .join('');
  return s.slice(0, 1).toLowerCase() + s.slice(1);
};
toCamelCase('some_database_field_name'); // 'someDatabaseFieldName'
toCamelCase('Some label that needs to be camelized'); // 'someLabelThatNeedsToBeCamelized'
toCamelCase('some-javascript-property'); // 'someJavascriptProperty'
toCamelCase('some-mixed_string with spaces_underscores-and-hyphens'); // 'someMixedStringWithSpacesUnderscoresAndHyphens'

toKebabCase

Converts a string to kebab case.

Break the string into words and combine them adding - as a separator, using a regexp.

const toKebabCase = str =>
  str &&
  str
    .match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
    .map(x => x.toLowerCase())
    .join('-');
toKebabCase('camelCase'); // 'camel-case'
toKebabCase('some text'); // 'some-text'
toKebabCase('some-mixed_string With spaces_underscores-and-hyphens'); // 'some-mixed-string-with-spaces-underscores-and-hyphens'
toKebabCase('AllThe-small Things'); // "all-the-small-things"
toKebabCase('IAmListeningToFMWhileLoadingDifferentURLOnMyBrowserAndAlsoEditingSomeXMLAndHTML'); // "i-am-listening-to-fm-while-loading-different-url-on-my-browser-and-also-editing-xml-and-html"

toSnakeCase

Converts a string to snake case.

Break the string into words and combine them adding _ as a separator, using a regexp.

const toSnakeCase = str =>
  str &&
  str
    .match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
    .map(x => x.toLowerCase())
    .join('_');
toSnakeCase('camelCase'); // 'camel_case'
toSnakeCase('some text'); // 'some_text'
toSnakeCase('some-mixed_string With spaces_underscores-and-hyphens'); // 'some_mixed_string_with_spaces_underscores_and_hyphens'
toSnakeCase('AllThe-small Things'); // "all_the_smal_things"
toSnakeCase('IAmListeningToFMWhileLoadingDifferentURLOnMyBrowserAndAlsoEditingSomeXMLAndHTML'); // "i_am_listening_to_fm_while_loading_different_url_on_my_browser_and_also_editing_some_xml_and_html"

truncateString

Truncates a string up to a specified length.

Determine if the string's length is greater than num. Return the string truncated to the desired length, with '...' appended to the end or the original string.

const truncateString = (str, num) =>
  str.length > num ? str.slice(0, num > 3 ? num - 3 : num) + '...' : str;
truncateString('boomerang', 7); // 'boom...'

unescapeHTML

Unescapes escaped HTML characters.

Use String.replace() with a regex that matches the characters that need to be unescaped, using a callback function to replace each escaped character instance with its associated unescaped character using a dictionary (object).

const unescapeHTML = str =>
  str.replace(
    /&amp;|&lt;|&gt;|&#39;|&quot;/g,
    tag =>
      ({
        '&amp;': '&',
        '&lt;': '<',
        '&gt;': '>',
        '&#39;': "'",
        '&quot;': '"'
      }[tag] || tag)
  );
unescapeHTML('&lt;a href=&quot;#&quot;&gt;Me &amp; you&lt;/a&gt;'); // '<a href="#">Me & you</a>'

URLJoin

Joins all given URL segments together, then normalizes the resulting URL.

Use String.join('/') to combine URL segments, then a series of String.replace() calls with various regexps to normalize the resulting URL (remove double slashes, add proper slashes for protocol, remove slashes before parameters, combine parameters with '&' and normalize first parameter delimiter).

const URLJoin = (...args) =>
  args
    .join('/')
    .replace(/[\/]+/g, '/')
    .replace(/^(.+):\//, '$1://')
    .replace(/^file:/, 'file:/')
    .replace(/\/(\?|&|#[^!])/g, '$1')
    .replace(/\?/g, '&')
    .replace('&', '?');
URLJoin('http://www.google.com', 'a', '/b/cd', '?foo=123', '?bar=foo'); // 'http://www.google.com/a/b/cd?foo=123&bar=foo'

words

Converts a given string into an array of words.

Use String.split() with a supplied pattern (defaults to non-alpha as a regexp) to convert to an array of strings. Use Array.filter() to remove any empty strings. Omit the second argument to use the default regexp.

const words = (str, pattern = /[^a-zA-Z-]+/) => str.split(pattern).filter(Boolean);
words('I love javaScript!!'); // ["I", "love", "javaScript"]
words('python, javaScript & coffee'); // ["python", "javaScript", "coffee"]

Type

getType

Returns the native type of a value.

Returns lowercased constructor name of value, "undefined" or "null" if value is undefined or null.

const getType = v =>
  v === undefined ? 'undefined' : v === null ? 'null' : v.constructor.name.toLowerCase();
getType(new Set([1, 2, 3])); // 'set'

is

Checks if the provided value is of the specified type (doesn't work with literals).

Use the instanceof operator to check if the provided value is of the specified type.

const is = (type, val) => val instanceof type;
is(Array, [1]); // true
is(ArrayBuffer, new ArrayBuffer()); // true
is(Map, new Map()); // true
is(RegExp, /./g); // true
is(Set, new Set()); // true
is(WeakMap, new WeakMap()); // true
is(WeakSet, new WeakSet()); // true
is(String, ''); // false
is(String, new String('')); // true
is(Number, 1); // false
is(Number, new Number(1)); // true
is(Boolean, true); // false
is(Boolean, new Boolean(true)); // true

isArrayLike

Checks if the provided argument is array-like (i.e. is iterable).

Use the spread operator (...) to check if the provided argument is iterable inside a try... catch block and the comma operator (,) to return the appropriate value.

const isArrayLike = val => {
  try {
    return [...val], true;
  } catch (e) {
    return false;
  }
};
isArrayLike(document.querySelectorAll('.className')); // true
isArrayLike('abc'); // true
isArrayLike(null); // false

isBoolean

Checks if the given argument is a native boolean element.

Use typeof to check if a value is classified as a boolean primitive.

const isBoolean = val => typeof val === 'boolean';
isBoolean(null); // false
isBoolean(false); // true

isFunction

Checks if the given argument is a function.

Use typeof to check if a value is classified as a function primitive.

const isFunction = val => typeof val === 'function';
isFunction('x'); // false
isFunction(x => x); // true

isNil

Returns true if the specified value is null or undefined, false otherwise.

Use the strict equality operator to check if the value and of val are equal to null or undefined.

const isNil = val => val === undefined || val === null;
isNil(null); // true
isNil(undefined); // true

isNull

Returns true if the specified value is null, false otherwise.

Use the strict equality operator to check if the value and of val are equal to null.

const isNull = val => val === null;
isNull(null); // true

isNumber

Checks if the given argument is a number.

Use typeof to check if a value is classified as a number primitive.

const isNumber = val => typeof val === 'number';
isNumber('1'); // false
isNumber(1); // true

isObject

Returns a boolean determining if the passed value is an object or not.

Uses the Object constructor to create an object wrapper for the given value. If the value is null or undefined, create and return an empty object. Οtherwise, return an object of a type that corresponds to the given value.

const isObject = obj => obj === Object(obj);
isObject([1, 2, 3, 4]); // true
isObject([]); // true
isObject(['Hello!']); // true
isObject({ a: 1 }); // true
isObject({}); // true
isObject(true); // false

isPlainObject

Checks if the provided value is an bbject created by the Object constructor.

Check if the provided value is truthy, use typeof to check if it is an object and Object.constructor to make sure the constructor is equal to Object.

const isPlainObject = val => !!val && typeof val === 'object' && val.constructor === Object;
isPlainObject({ a: 1 }); // true
isPlainObject(new Map()); // false

isPrimitive

Returns a boolean determining if the passed value is primitive or not.

Use Array.includes() on an array of type strings which are not primitive, supplying the type using typeof. Since typeof null evaluates to 'object', it needs to be directly compared.

const isPrimitive = val => !['object', 'function'].includes(typeof val) || val === null;
isPrimitive(null); // true
isPrimitive(50); // true
isPrimitive('Hello!'); // true
isPrimitive(false); // true
isPrimitive(Symbol()); // true
isPrimitive([]); // false

isPromiseLike

Returns true if an object looks like a Promise, false otherwise.

Check if the object is not null, its typeof matches either object or function and if it has a .then property, which is also a function.

const isPromiseLike = obj =>
  obj !== null &&
  (typeof obj === 'object' || typeof obj === 'function') &&
  typeof obj.then === 'function';
isPromiseLike({
  then: function() {
    return '';
  }
}); // true
isPromiseLike(null); // false
isPromiseLike({}); // false

isString

Checks if the given argument is a string.

Use typeof to check if a value is classified as a string primitive.

const isString = val => typeof val === 'string';
isString('10'); // true

isSymbol

Checks if the given argument is a symbol.

Use typeof to check if a value is classified as a symbol primitive.

const isSymbol = val => typeof val === 'symbol';
isSymbol(Symbol('x')); // true

isUndefined

Returns true if the specified value is undefined, false otherwise.

Use the strict equality operator to check if the value and of val are equal to undefined.

const isUndefined = val => val === undefined;
isUndefined(undefined); // true

isValidJSON

Checks if the provided argument is a valid JSON.

Use JSON.parse() and a try... catch block to check if the provided argument is a valid JSON.

const isValidJSON = obj => {
  try {
    JSON.parse(obj);
    return true;
  } catch (e) {
    return false;
  }
};
isValidJSON('{"name":"Adam","age":20}'); // true
isValidJSON('{"name":"Adam",age:"20"}'); // false
isValidJSON(null); // true

Utility

cloneRegExp

Clones a regular expression.

Use new RegExp(), RegExp.source and RegExp.flags to clone the given regular expression.

const cloneRegExp = regExp => new RegExp(regExp.source, regExp.flags);
const regExp = /lorem ipsum/gi;
const regExp2 = cloneRegExp(regExp); // /lorem ipsum/gi

coalesce

Returns the first non-null/undefined argument.

Use Array.find() to return the first non null/undefined argument.

const coalesce = (...args) => args.find(_ => ![undefined, null].includes(_));
coalesce(null, undefined, '', NaN, 'Waldo'); // ""

coalesceFactory

Returns a customized coalesce function that returns the first argument that returns true from the provided argument validation function.

Use Array.find() to return the first argument that returns true from the provided argument validation function.

const coalesceFactory = valid => (...args) => args.find(valid);
const customCoalesce = coalesceFactory(_ => ![null, undefined, '', NaN].includes(_));
customCoalesce(undefined, null, NaN, '', 'Waldo'); // "Waldo"

extendHex

Extends a 3-digit color code to a 6-digit color code.

Use Array.map(), String.split() and Array.join() to join the mapped array for converting a 3-digit RGB notated hexadecimal color-code to the 6-digit form. Array.slice() is used to remove # from string start since it's added once.

const extendHex = shortHex =>
  '#' +
  shortHex
    .slice(shortHex.startsWith('#') ? 1 : 0)
    .split('')
    .map(x => x + x)
    .join('');
extendHex('#03f'); // '#0033ff'
extendHex('05a'); // '#0055aa'

getURLParameters

Returns an object containing the parameters of the current URL.

Use String.match() with an appropriate regular expression to get all key-value pairs, Array.reduce() to map and combine them into a single object. Pass location.search as the argument to apply to the current url.

const getURLParameters = url =>
  url
    .match(/([^?=&]+)(=([^&]*))/g)
    .reduce((a, v) => ((a[v.slice(0, v.indexOf('='))] = v.slice(v.indexOf('=') + 1)), a), {});
getURLParameters('http://url.com/page?name=Adam&surname=Smith'); // {name: 'Adam', surname: 'Smith'}

hexToRGBadvanced

Converts a color code to a rgb() or rgba() string if alpha value is provided.

Use bitwise right-shift operator and mask bits with & (and) operator to convert a hexadecimal color code (with or without prefixed with #) to a string with the RGB values. If it's 3-digit color code, first convert to 6-digit version. If an alpha value is provided alongside 6-digit hex, give rgba() string in return.

const hexToRGB = hex => {
  let alpha = false,
    h = hex.slice(hex.startsWith('#') ? 1 : 0);
  if (h.length === 3) h = [...h].map(x => x + x).join('');
  else if (h.length === 8) alpha = true;
  h = parseInt(h, 16);
  return (
    'rgb' +
    (alpha ? 'a' : '') +
    '(' +
    (h >>> (alpha ? 24 : 16)) +
    ', ' +
    ((h & (alpha ? 0x00ff0000 : 0x00ff00)) >>> (alpha ? 16 : 8)) +
    ', ' +
    ((h & (alpha ? 0x0000ff00 : 0x0000ff)) >>> (alpha ? 8 : 0)) +
    (alpha ? `, ${h & 0x000000ff}` : '') +
    ')'
  );
};
hexToRGB('#27ae60ff'); // 'rgba(39, 174, 96, 255)'
hexToRGB('27ae60'); // 'rgb(39, 174, 96)'
hexToRGB('#fff'); // 'rgb(255, 255, 255)'

httpGet

Makes a GET request to the passed URL.

Use XMLHttpRequest web api to make a get request to the given url. Handle the onload event, by calling the given callback the responseText. Handle the onerror event, by running the provided err function. Omit the third argument, err, to log errors to the console's error stream by default.

const httpGet = (url, callback, err = console.error) => {
  const request = new XMLHttpRequest();
  request.open('GET', url, true);
  request.onload = () => callback(request.responseText);
  request.onerror = () => err(request);
  request.send();
};
httpGet(
  'https://jsonplaceholder.typicode.com/posts/1',
  console.log
); /* 
Logs: {
  "userId": 1,
  "id": 1,
  "title": "sunt aut facere repellat provident occaecati excepturi optio reprehenderit",
  "body": "quia et suscipit\nsuscipit recusandae consequuntur expedita et cum\nreprehenderit molestiae ut ut quas totam\nnostrum rerum est autem sunt rem eveniet architecto"
}
*/

httpPost

Makes a POST request to the passed URL.

Use XMLHttpRequest web api to make a post request to the given url. Set the value of an HTTP request header with setRequestHeader method. Handle the onload event, by calling the given callback the responseText. Handle the onerror event, by running the provided err function. Omit the third argument, data, to send no data to the provided url. Omit the fourth argument, err, to log errors to the console's error stream by default.

const httpPost = (url, data, callback, err = console.error) => {
  const request = new XMLHttpRequest();
  request.open('POST', url, true);
  request.setRequestHeader('Content-type', 'application/json; charset=utf-8');
  request.onload = () => callback(request.responseText);
  request.onerror = () => err(request);
  request.send(data);
};
const newPost = {
  userId: 1,
  id: 1337,
  title: 'Foo',
  body: 'bar bar bar'
};
const data = JSON.stringify(newPost);
httpPost(
  'https://jsonplaceholder.typicode.com/posts',
  data,
  console.log
); /*
Logs: {
  "userId": 1,
  "id": 1337,
  "title": "Foo",
  "body": "bar bar bar"
}
*/
httpPost(
  'https://jsonplaceholder.typicode.com/posts',
  null, //does not send a body
  console.log
); /*
Logs: {
  "id": 101
}
*/

parseCookie

Parse an HTTP Cookie header string and return an object of all cookie name-value pairs.

Use String.split(';') to separate key-value pairs from each other. Use Array.map() and String.split('=') to separate keys from values in each pair. Use Array.reduce() and decodeURIComponent() to create an object with all key-value pairs.

const parseCookie = str =>
  str
    .split(';')
    .map(v => v.split('='))
    .reduce((acc, v) => {
      acc[decodeURIComponent(v[0].trim())] = decodeURIComponent(v[1].trim());
      return acc;
    }, {});
parseCookie('foo=bar; equation=E%3Dmc%5E2'); // { foo: 'bar', equation: 'E=mc^2' }

prettyBytes

Converts a number in bytes to a human-readable string.

Use an array dictionary of units to be accessed based on the exponent. Use Number.toPrecision() to truncate the number to a certain number of digits. Return the prettified string by building it up, taking into account the supplied options and whether it is negative or not. Omit the second argument, precision, to use a default precision of 3 digits. Omit the third argument, addSpace, to add space between the number and unit by default.

const prettyBytes = (num, precision = 3, addSpace = true) => {
  const UNITS = ['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
  if (Math.abs(num) < 1) return num + (addSpace ? ' ' : '') + UNITS[0];
  const exponent = Math.min(Math.floor(Math.log10(num < 0 ? -num : num) / 3), UNITS.length - 1);
  const n = Number(((num < 0 ? -num : num) / 1000 ** exponent).toPrecision(precision));
  return (num < 0 ? '-' : '') + n + (addSpace ? ' ' : '') + UNITS[exponent];
};
prettyBytes(1000); // "1 KB"
prettyBytes(-27145424323.5821, 5); // "-27.145 GB"
prettyBytes(123456789, 3, false); // "123MB"

randomHexColorCode

Generates a random hexadecimal color code.

Use Math.random to generate a random 24-bit(6x4bits) hexadecimal number. Use bit shifting and then convert it to an hexadecimal String using toString(16).

const randomHexColorCode = () => {
  let n = ((Math.random() * 0xfffff) | 0).toString(16);
  return '#' + (n.length !== 6 ? ((Math.random() * 0xf) | 0).toString(16) + n : n);
};
randomHexColorCode(); // "#e34155"

RGBToHex

Converts the values of RGB components to a color code.

Convert given RGB parameters to hexadecimal string using bitwise left-shift operator (<<) and toString(16), then String.padStart(6,'0') to get a 6-digit hexadecimal value.

const RGBToHex = (r, g, b) => ((r << 16) + (g << 8) + b).toString(16).padStart(6, '0');
RGBToHex(255, 165, 1); // 'ffa501'

serializeCookie

Serialize a cookie name-value pair into a Set-Cookie header string.

Use template literals and encodeURIComponent() to create the appropriate string.

const serializeCookie = (name, val) => `${encodeURIComponent(name)}=${encodeURIComponent(val)}`;
serializeCookie('foo', 'bar'); // 'foo=bar'

timeTaken

Measures the time taken by a function to execute.

Use console.time() and console.timeEnd() to measure the difference between the start and end times to determine how long the callback took to execute.

const timeTaken = callback => {
  console.time('timeTaken');
  const r = callback();
  console.timeEnd('timeTaken');
  return r;
};
timeTaken(() => Math.pow(2, 10)); // 1024, (logged): timeTaken: 0.02099609375ms

toDecimalMark

Use toLocaleString() to convert a float-point arithmetic to the Decimal mark form. It makes a comma separated string from a number.

const toDecimalMark = num => num.toLocaleString('en-US');
toDecimalMark(12305030388.9087); // "12,305,030,388.909"

toOrdinalSuffix

Adds an ordinal suffix to a number.

Use the modulo operator (%) to find values of single and tens digits. Find which ordinal pattern digits match. If digit is found in teens pattern, use teens ordinal.

const toOrdinalSuffix = num => {
  const int = parseInt(num),
    digits = [int % 10, int % 100],
    ordinals = ['st', 'nd', 'rd', 'th'],
    oPattern = [1, 2, 3, 4],
    tPattern = [11, 12, 13, 14, 15, 16, 17, 18, 19];
  return oPattern.includes(digits[0]) && !tPattern.includes(digits[1])
    ? int + ordinals[digits[0] - 1]
    : int + ordinals[3];
};
toOrdinalSuffix('123'); // "123rd"

validateNumber

Returns true if the given value is a number, false otherwise.

Use !isNaN() in combination with parseFloat() to check if the argument is a number. Use isFinite() to check if the number is finite. Use Number() to check if the coercion holds.

const validateNumber = n => !isNaN(parseFloat(n)) && isFinite(n) && Number(n) == n;
validateNumber('10'); // true

yesNo

Returns true if the string is y/yes or false if the string is n/no.

Use RegExp.test() to check if the string evaluates to y/yes or n/no. Omit the second argument, def to set the default answer as no.

const yesNo = (val, def = false) =>
  /^(y|yes)$/i.test(val) ? true : /^(n|no)$/i.test(val) ? false : def;
yesNo('Y'); // true
yesNo('yes'); // true
yesNo('No'); // false
yesNo('Foo', true); // true