Flatten Array

flatten.js

function flatten(input) {
  'use strict';
  if (!Array.isArray(input))  // need polyfill
    return input;

  var ret = [];
  (function inner(input, ret) {
    if (input.length === 0)
      return ret;
    var car = input.shift(0);
    if (!Array.isArray(car)) {
      ret.push(car);
    } else {
      inner(car, ret);
    }
    inner(input, ret);
  })(input, ret);

  return ret;
}

console.log(flatten([1, [2, 3, [4, 5], [6, 7]], [8, 9], 10]));