function() {
    this.a = "a"; // This obc creates a variable called "a" in this function, like a field.
}

class A {

    test() {
        this.a = "a2"; // Same here. The "a" variable in the other function is not the same as this one. But...
    }
}

class B {

    test() {
        this.a = function() {
            this.a = "a3"; // HOLD UP! the "this" identifier IS NOT referencing the class. It is referencing the function, like how i showed it in the first example!!!!!
        }; // "a" is now a function. you can call it if you want.
        this.a();
    }

    test2() { // If you want to fix that problem you need to do this:

        this.a = function() {
            this.a = "a3";
        }.bind(this); // This bind function can be used for a ton of things, but you are passing in an instance of "this" that you want to use. You can also do stuff like:

        this.b = function(myName) {
            console.log(myName); // "doggo"
        }.bind(this, "doggo");

        // If you aren't really referencing "this", you can just pass in null as the first parameter
    }
}