Flatten an Array with JS

flatten.js

const assert = require('assert');
const input1 = [1, [2, 3], Object, [], 5, 6];
const input2 = [1, 2, [3, 4, [5, 6, 7], 8], 9, [[10]]];
const input3 = [1, 2, 3, 4, 5];

function flatten(arr) {
  return Array.isArray(arr) ? [].concat.apply([], arr.map(flatten)) : arr;
}

assert.deepEqual(flatten(input1), [1, 2, 3, Object, 5, 6]);
assert.deepEqual(flatten(input2), [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
assert.deepEqual(flatten(input3), [1, 2, 3, 4, 5]);