Add IsEmpty shim to JS Array

Question

Update

Thanks for your help guys, I was unaware of the things to be aware of when using lambda (fat arrow) functions.

if (window.Array && !Array.prototype.isEmpty)
{
 Array.prototype.isEmpty = function ()
 {
  return this.length === 0;
 };
}

console.log("[]", [].isEmpty());
console.log("[1]", [1].isEmpty());

This works now.

---

Original Question

I have been trying to add the functionality of isEmpty to the Array prototype in javascript to both make my code a little cleaner and also because I want to learn how to do it properly.

I understand that it is advisable to not add your own methods to the existing objects and to make your own, but at this point, I really want to figure out where I am going astray.

I also understand that you can test for an empty array by just doing array.length === 0.

I have been running into issues where my this is referring to the window object instead of the calling array.

This is my code

if (window.Array && !Array.prototype.isEmpty)
{
    Array.prototype.isEmpty = (callback, thisArg) =>
    {
     return this.length === 0;
    };
}

console.log("[]", [].isEmpty());
console.log("[1]", [1].isEmpty());

Because this is a reference to the window object, this.length is false so no matter what array is calling it, this function returns true.

How do I get this to refer to the calling object instead of window?

I based on this forEach shim for NodeList, I had tried to change my isEmpty function to emulate it because I saw that the forEach was using a callback and my shim wasn't.

NodeList.forEach shim:

if (window.NodeList && !NodeList.prototype.forEach)
{
    NodeList.prototype.forEach = (callback, thisArg) =>
    {
        thisArg = thisArg || window;
        for (let i = 0; i < this.length; i++)
        {
            callback.call(thisArg, this[i], i, this);
        }
    };
}

Seeing that, this is what I tried to change my function to

if (window.Array && !Array.prototype.isEmpty)
{
    Array.prototype.isEmpty = (callback, thisArg) =>
    {
        thisArg = thisArg || window;
        callback.call(thisArg, this.length === 0, 0, this);
    };
}

console.log("[]", [].isEmpty());
console.log("[1]", [1].isEmpty());

Unfortunately, this results in an error as the callback function is not defined, I was hoping to send an empty function.

The difference between the two functions is that in the NodeList.forEach this is a reference to the NodeList that is invoking forEach instead of a reference to window.

From what I can tell, the difference, in the code, between the working NodeList.forEach and my failing Array.isEmpty is that with forEach you supply it with a function to act on it, but with isEmpty it's just an empty function call.

I will admit that I don't understand callback functions as well as I would like so I am flailing a bit here. I have tried looking up implementations of isEmpty for JS but most of the responses are just use array.length === 0, while I will agree this is a good solution, I am trying to push my knowledge boundaries.

Thank you for any insight you can give.