[JavaScript] Flatten an array.

flatten.js

const assert = require('assert')

// Null is a value, undefined is undefined.
// That's why we have to consider undefined case.

// Example array of nested elements
const array = [1, [2, 3], [[4, 5]], [undefined], [6, 7], [[[[]]]], [8], [[[[[9]]]]]];
// Expected output for `flatten(array)`
const expected = [1, 2, 3, 4, 5, 6, 7, 8, 9];

const flatten = function (array, result = []) {
  //
  for (let i = 0; i < array.length; i++) {
    const value = array[i];
    // If our value is array, we have to dig down.
    if (Array.isArray(value)) {
      flatten(value, result);
    } else {
      if (value !== undefined) {
        result.push(value);
      }
    }
  }
  return result;
}

assert.deepStrictEqual(flatten(array), expected);