flatten-array.js

// original array
var array = [[1,2,[3]],4];

/**
 * Flatten function for arrays with unlimited depth
 */
const flatten = array => {
   // if input is not array, throw error
  if (!Array.isArray(array)) throw (new Error('This function can only be used on arrays'));
  
  // loop through array
  while (array.find(el => Array.isArray(el))) array = Array.prototype.concat(...array);
  
  return array
}

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