Flatten Array

flatten.js

// this implementation preserves the original arr
function flatten(arr) {
  var result = [];
  var queue = [];
  var pointer = 0;

  while(pointer < arr.length) {
    queue.push(arr[pointer]);
    pointer += 1;

    while(queue.length > 0) {
      var current = queue.shift();

      if (Array.isArray(current)) {
        for (var i = current.length - 1; i >= 0; i--) {
          queue.unshift(current[i]);
        }
      } else {
        result.push(current);
      }
    }
  }

  return result;
}