Compact an array or object in JavaScript

The term "compact" is used to describe the process of removing all falsy values from an array or object. Generally speaking, you can use the Boolean function to filter out all falsy values (false, null, 0, '', undefined, and NaN), compacting the array or object in the process.

Compact an array

Compacting an array is as simple as using Array.prototype.filter() combined with the Boolean function.

const compact = arr => arr.filter(Boolean);

compact([0, 1, false, 2, '', 3, 'a', 'e' * 23, NaN, 's', 34]);
// [ 1, 2, 3, 'a', 's', 34 ]

Compact an object

Compacting an object is fairly similar to the array, but you have to use Object.entries() to iterate over the object before filtering its key-value pairs.

const compact = obj =>
  Object.fromEntries(
    Object.entries(obj).filter(([key, value]) => Boolean(value))
  );

compact({ a: 0, b: 1, c: false, d: '', e: 2, f: 'a', g: 'e' * 23, h: NaN });
// { b: 1, e: 2, f: 'a' }

Deep compact an array or object

Up until this point, we've only shallowly compacted arrays and objects. If you want to deeply compact an array or object, you can use recursion to compact all nested arrays and objects.

First, initialize the iterable data, using Array.isArray(), Array.prototype.filter() and Boolean for arrays in order to avoid sparse arrays. Using Object.entries() and Array.prototype.reduce(), we can iterate over each key with an appropriate initial value. We can then use Boolean to determine the truthiness of each key's value and add it to the accumulator if it's truthy. Finally, we can use typeof to determine if a given value is an object and call the function again to deeply compact it.

const deepCompact = val => {
  const data = Array.isArray(val) ? val.filter(Boolean) : val;
  return Object.entries(data).reduce(
    (acc, [key, value]) => {
      if (Boolean(value))
        acc[key] = typeof value === 'object' ? deepCompact(value) : value;
      return acc;
    },
    Array.isArray(val) ? [] : {}
  );
};

const obj = {
  a: null,
  b: false,
  c: true,
  d: 0,
  e: 1,
  f: '',
  g: 'a',
  h: [null, false, '', true, 1, 'a', { i: 0, j: 2}],
  k: { l: 0, m: false, n: 'a', o: [0, 1] }
};
/*
{
  c: true,
  e: 1,
  g: 'a',
  h: [ true, 1, 'a', { j: 2 } ],
  k: { n: 'a', o: [1] }
}
*/