deepFlatten

Deep flattens an array.

• Use recursion.
• Use Array.prototype.concat() with an empty array ([]) and the spread operator (...) to flatten an array.
• Recursively flatten each element that is an array.

const deepFlatten = arr =>
  [].concat(...arr.map(v => (Array.isArray(v) ? deepFlatten(v) : v)));

deepFlatten([1, [2], [[3], 4], 5]); // [1, 2, 3, 4, 5]