Flatten an arran in JS

flatten.js

function flatten(arr) {
  var i = 0;
  while (i < arr.length) {
   arr = arr.reduce(function (prev, curr) {
      return prev.concat(curr);
  }, []);
    i++;
  }
  return arr;
}

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