Q: Flatten Array

Recursive Solution

function flatten(arr) {
  if (!Array.isArray(arr)) {
    return [arr];
  }

  var array = [];
  for (var i = 0; i < arr.length; i++) {
    array = array.concat(flatten(arr[i]));
  }
  return array;
}

flatten([1, [2, [3]], [4]]); // => [1,2,3,4]

Iterative Solution

function flatten(arr) {
  var array = [];
  while (arr.length) {
    var value = arr.shift();
    if (Array.isArray(value)) {
      // this line preserve the order
      arr = value.concat(arr);
    } else {
      array.push(value);
    }
  }
  return array;
}

flatten([1, [2, [3]], [4]]); // => [1,2,3,4]