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



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.

Ensure the value is not undefined or null using Array.prototype.includes(), and compare the constructor property on the value with type to check if the provided value is of the specified type.

const is = (type, val) => ![, null].includes(val) && val.constructor === 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, ''); // true
is(String, new String('')); // true
is(Number, 1); // true
is(Number, new Number(1)); // true
is(Boolean, true); // true
is(Boolean, new Boolean(true)); // true

isArrayLike

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

Check if the provided argument is not null and that its Symbol.iterator property is a function.

const isArrayLike = obj => obj != null && typeof obj[Symbol.iterator] === 'function';
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

isEmpty

Returns true if the a value is an empty object, collection, map or set, has no enumerable properties or is any type that is not considered a collection.

Check if the provided value is null or if its length is equal to 0.

const isEmpty = val => val == null || !(Object.keys(val) || val).length;
isEmpty(new Map()); // true
isEmpty(new Set()); // true
isEmpty([]); // true
isEmpty({}); // true
isEmpty(''); // true
isEmpty([1, 2]); // false
isEmpty({ a: 1, b: 2 }); // false
isEmpty('text'); // false
isEmpty(123); // true - type is not considered a collection
isEmpty(true); // true - type is not considered a collection

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

isObjectLike

Checks if a value is object-like.

Check if the provided value is not null and its typeof is equal to 'object'.

const isObjectLike = val => val !== null && typeof val === 'object';
isObjectLike({}); // true
isObjectLike([1, 2, 3]); // true
isObjectLike(x => x); // false
isObjectLike(null); // false

isPlainObject

Checks if the provided value is an object 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.

Create an object from val and compare it with val to determine if the passed value is primitive (i.e. not equal to the created object).

const isPrimitive = val => Object(val) !== val;
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. Only works for string primitives.

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