nested function cant find itself in Typescript - html

Question

I have the following code:
//Main Entry Point.
start() {
this.init();
this.gameLoop();
}
//Init, runs only once.
init() {
let initalEntity1: Entity = new Entity(10, 10, Gender.female);
let initalEntity2: Entity = new Entity(60, 60, Gender.male);
console.log("There are " + this.Entities.length + " Items in the entities Array")
this.Entities.push(initalEntity1, initalEntity2);
console.log("There are " + this.Entities.length + " Items in the entities Array")
}
gameLoop() {
console.log("Performing a Game Loop");
requestAnimationFrame(this.gameLoop);
//MAIN LOOP THROUGH ENTITIES
for (let i in this.Entities) {
this.Render.drawEntitysToScreen(this.Entities[i].EntityPosition, this.Entities[i].gender);
}
}
It enters into start() fine, and also performs all of the init() functionality. it them proceeds onto gameloop() which it will run once, however the line requestAnimationFrame(this.gameLoop); which is ment to retrigger the function to be called as a Canvas frame is causing the following error:
TypeError: this is undefined
trying to requestAnimationFrame(gameLoop); but it causes the typescript compiler to get upset...

Answer 1

This is due to how this binding works in javascript. The way by which you are passing this.gameLoop to requestAnimationFrame is essentially passing an unbound gameLoop function, and so when it is called, it has lost reference to its this.
There are a number of possible solutions to this problem:
You can bind this.gameLoop to this inside of the class constructor, like so:
constructor() {
this.gameLoop = this.gameLoop.bind(this);
}
You can bind this.gameLoop to this as part of the gameLoop method definition. Rather than defining gameLoop like
gameLoop() {
If you instead use
gameLoop = () => {
it will be automatically bound to this. This is a property of using the fat arrow for function declarations: it automatically performs binding to the this that exists at the function declaration.
You can change how you pass gameLoop to requestAnimationFrame:
requestAnimationFrame(() => this.gameLoop());
This again takes advantage of the automatic this binding performed by the arrow function, but instead of doing it as part of the class method declaration you can simply do it lazily at the time you need it to be bound.
Note, however, that doing it this way does mean that a new function will be created each time gameLoop is called.

Answer 2

When you call this.gameLoop() within start, the value for this inside gameLoop's body will be the class gameLoop belongs to, because you call gameLoop as a property of this (the class).
When you pass a function reference its value for this might be anything when the function is called from somewhere else.
Solution 1 | Using Function.prototype.bind
bind the value for this as you give the function to requestAnimationFrame. By doing this you explicitly say:
Let the argument to bind be this inside any call to gameLoop, regardless of how it is called, or where it is called from.
requestAnimationFrame(this.gameLoop.bind(this));
Note that bind returns you a new function, so the original gameLoop function that is still a property of your class remains unchanged.
Solution 2 | Using an arrow function
Define an arrow function to eventually execute the call to gameLoop instead of requestAnimationFrame. The this value within arrow functions is static, and is inherited from the execution context enclosing the function declaration.
requestAnimationFrame(() => this.gameLoop());