This tutorial is a cookbook of my favorite JavaScript looping techniques and to a lesser degree, object introspection. It should prove useful to anyone new to JavaScript, especially if you are coming from languages with for...in (python in particular).

I'm doing this in my browser JavaScript console.

Looping for values

Looping through elements of an array is easy in JavaScript. Use for...of!

> let arr = [5, 10, 15, 20]
> for (let i of arr){
    console.log(i)
  }
5
10
15
20

for...of returns the value of the loop iteration. It's the right tool for this operation.

Looping for index

If you need to track the index of a loop, do this:

> for (let i=0; i < arr.length; i++){
    console.log(i)
  }
0
1
2
3

This technique is reliable and acts predictably. I've heard that this method runs more quickly than for...of, but JavaScript is very fast these days. Any slowdowns your code might have is probably due to latency or other bottlenecks rather than which JavaScript looping technique you choose to implement.

What's the difference between for...of and for...in?

Let's use both techniques on the same array and see what we get. First, the for...of combination:

> let arr = [5, 10, 15, 20]
> for (let i of arr){
    console.log(i)
  }
5
10
15
20

Now, the for...in combination, which isn't recommended:

> let arr = [5, 10, 15, 20]
> for (let i in arr){
    console.log(i)
  }
0
1
2
3

It appears that for...in returns the index of the loop iteration, right? What's wrong with that?

Read on!

Problems using for...in with arrays

for...in can fool you into thinking it's the right tool for iterating over arrays for indexes instead of the more verbose approach of (let i=0; i < arr.length; i++). This appeared to work:

> let arr = [5, 10, 15, 20]
> for (let i in arr){
  console.log(i)
}
0
1
2
3

But let's change things up a bit with some prototype manipulation:

> Array.prototype.icecream = function(){
  console.log("Ice cream!")
}
> let arr = [5, 10, 15, 20]
> arr
(4) [5, 10, 15, 20]

The array has been proven to have four elements. Now let's iterate over the array using for...in:

> for (let i in array){
  console.log(i)
}
0
1
2
3
icecream

Where did the icecream value come from? icecream function wasn't in the array but for...in called it anyway. What just happened?!?

What's going on is that the for...in is enumerating over the properties of the array, not the values or index. Even if you avoid using prototypes, the same cannot be said for any library installed from NPM. Finally, there is no guarantee that the elements will be returned for...in in numeric order.

What about forEach loops?

The forEach loop requires a callback, making it a slightly advanced enough method of writing for loops in JavaScript. I'll cover that in a future article.

Summary

1. Use for...of for iterating over the values of an array.
2. Use (let i=0; i < arr.length; i++) for enumerating over the index of an array.
3. Avoid using for...in for iterating over arrays in any capacity.