'For In' loop only for objects?

This post is quite old but in the meantime, I've got some more information to share about this subject. As we know, Dave said in the video 'For in Loop' are only for objects. He is right when he tells it as Arrays are as well objects in Javascript. That's the first point.

Lastly, we should indeed avoid using for in loops with arrays. Why ? Because if we use some libraries of frameworks which extends the functionnalities of arrays, by adding methods to them or properties. The for in loop will loop as well through these added methods and properties, as this kind of loop iterates over properties of an object once again.

An example is worth a thoudand words.

Array.prototype.myCustomFeature = 'I added my custom properties to the Array Object';

var arr = ["Treehouse","Students","Teacher"];

for (var prop in arr){
    console.log(prop+ ': '+arr[prop]);

}

The output in the console will be this :

0: Treehouse 1: Students 2: Teacher myCustomFeature: I added my custom properties to the Array Object

I guess you now see why that should be avoided. Instead we should stick with classical loop to iterate over arrays. To be clear, like this

Array.prototype.myCustomFeature = 'I added my custom properties to the Array Object';
var arr = ["Treehouse","Students","Teacher"];

for (var i = 0;  i<arr.length ; i++){
    console.log(i+ ': '+arr[i]);
}

As an output we get this:

0: Treehouse 1: Students 2: Teacher

In other words, we don't loop over the added properties and methods. I wanted to update this topic as that's something we should be aware of in my opinion.