david-oliveros-flatten-deep.js

/**
 * Recursively flattens an array.
 *
 * @param  {Array}  arr  Array to flatten.
 * @return {Array}       Flattened array.
 */
function flatten(arr) {
  var flattened = [];

  for (var i = 0, len = arr.length; i < len; i++) {
    if (Array.isArray(arr[i])) {
      flattened.push.apply(flattened, flatten(arr[i]));
    } else {
      flattened.push(arr[i]);
    }
  }

  return flattened;
}

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