Flatten out an array

flattenArray.js

/**
 * flatten out an array
 * @param  {Array} input
 * @param  {Array} [output=[]]
 * @return {Array}
 */
const flatten = (input, output = []) => {
  // current index
  let index = output.length;

  // loop through array values
  for (let i = 0; i < input.length; i++) {
    const value = input[i];

    // check if current value is an array
    if (Array.isArray(value)) {
      // recursive call to flat out the array value
      flatten(value, output);
    } else {
      // insert the value into output array
      if (value) output[index++] = value;
    }
  }

  return output;
}

// tests
console.log(flatten([1, 2, 3]));
// => [ 1, 2, 3 ]
console.log(flatten([1, 2, [[[3]]]]));
// => [ 1, 2, 3 ]
console.log(flatten([1, 2, [], [], [[[3]]]]));
// => [ 1, 2, 3 ]
console.log(flatten([1, [[2]], , [[], [4]], [[[3]]]]));
// => [ 1, 2, 4, 3 ]