Question on a for loop

Question on a for loop
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Hello! I had a question on a for loop for a solution for the steam roller challenge

The code is:

function steamrollArray(arr) {
var newArray = [];
var flatten = function(arg) {
  } else {
    for(var a in arg) {
  return newArray;

My question is how come in the for loop you have to use for(var a in arg) and instead cannot use something like for(var i = 0; i<arg.length; i++)? I understand everything else about the code.


hey @njanne19! Have you checked out the documentation for on MDN?


It is possible to use a for loop in your code. I did use a for loop in my code. However, I did nest it in a while loop.

Here is my code:

function steamrollArray(arr) {
  // I'm a steamroller, baby
  var isFlattened = false;
  while (isFlattened === false) {
    for (var i = 0; i < arr.length; i++) {
      if (Array.isArray(arr[i])) {
          arr = arr.reduce(function(a, b) { // Reduces the array by one dimension.
          return a.concat(b);
        }, []);
        i--; // Needed in order to check if element at this index is still an array.
    isFlattened = true;
  return arr;


@njanne19 - You should be able to use your version of the for loop also. Give it a try. Replace the for/in loop in your code with a standard for loop:

for (var i = 0; i<arg.length; i++) {

The for/in loop does the same thing as above. I think it is used because it takes less keystrokes to write. There are a few other loop types as well, there is also ‘while’ loops and ‘do/while’ loops that could be used to. Honestly, I think that you could use whatever you are most comfortable with. I tend to use the good old standard ‘for’ loop.


There’s also a pretty sweet way to solve this challenge with just recursion and spread operator in place of recursion + loops.

The loop is more commonly used on objects, it is used to iterate over an object’s properties, but in an arbitrary order - so i think a regular for loop or a for...of loop would be safer in the original example.

    function forIn(arr) {
      for(var idx in arr) { 
         console.log(idx, arr[idx]); // <--- (indexes, items)

    forIn(['a', 'b', 'c', 'd']);

    // logs: 0, 1, 2, 3 <--- indexes
    // logs: a, b, c, d <--- items

for...of loops are more applicable to iterating over an array’s elements:

    function forOf(arr) {
      for(var el of arr) { 
         console.log(el); // <--- (items)

    forOf(['a', 'b', 'c', 'd']);

    // logs: a, b, c, d

so this would be just as effective and potentially more reliable (at leas according to documentation):

    for(var a of arg) {
        flatten(a);  // does the same thing

BUT, if anyone is interested, the spread and recursion syntax is the shortest and most fun! Worth looking up, it’s out there.


Thank you for the help. All of your tips have worked well