A simple implementation of an in-place flatten function in javascript

inPlaceFlatten.js

function inPlaceFlatten(arr){
  for (var i = 0; i < arr.length; i++) {
    if(arr[i] instanceof Array){
      var temp = inPlaceFlatten(arr[i]);
      arr = arr.slice(0, i).concat(temp, arr.slice(i + 1));
      i += temp.length - 1;
    } 
  };
  return arr;
}
// console.log(inplaceFlatten([1,[2,3,[4, 5]],6])) -> [1,2,3,4,5,6]