this keyword in JavaScript

1. "this" keyword refers to the object which invoked the method 
when referenced inside a method. 
- This one is pretty simple, this is how this keyword works in every language.

2. "this" keyword refers to the global object when used inside a function, 
but is undefined when invoked in function in strict mode.
-- There is "use strict" statement that we write on top of a script which means, 
before this

According to ES6 classes, strict mode is always enabled and cannot be disabled.
It's a new feature of ECMAScript 5. 

It's just a string you put in your JavaScript files (either at the top of your file or inside of a function) that looks like this:

"use strict";

Putting it in your code now shouldn't cause any problems with current browsers as it's just a string. It may cause problems with your code in the future if your code violates the pragma. For instance, if you currently have foo = "bar" without defining foo first, your code will start failing...

In your case, this is the class:

class Animal {
    static planet = 'Earth';

    constructor(name, speed = 10) {
        this.name = name;
        this.speed = speed;
    }

    static compareSpeeds(a1, a2) {
        return a2.speed - a1.speed;
    }

    run() {
        console.log(`${this.name} runs at ${this.speed} mph`);
    }

    eat() {
        console.log(`${this.name} eats like an animal`);
    }

    describe() {
        this.eat();
        this.run();
    }
   
}

//this is the array of objects of above class
let arrayOfAnimals = [new Animal("generic animal", 10),
new Animal("fast generic animal", 20)];
arrayOfAnimals.sort(Animal.compareSpeeds);//sort from fastest to slowest
// arrayOfAnimals.forEach((a) => console.log('In ', Animal.planet, a.describe()));

//console.log(arrayOfAnimals.sort(Animal.compareSpeeds));
//console.log(arrayOfAnimals.length);

//Now to make this keyword work outside the class, we are using apply method.
//With apply() method, you can write a method that can be used on different objects.



const person = {
  describeOutput: function() { //this is a method inside the object
return this.eat() + " " + this.run();
}
}

for(let a of arrayOfAnimals) {
console.log(Animal.planet);
//console.log(a.describe());
      person.describeOutput.apply(a); //and here i am calling a method with apply on 
      //the object so that this keyword can work properly
}  

3. "this" keyword in Javascript refers to the DOM element when used inside an event fired on the DOM element.