Why can't I access `userService` variable after express instance running in Nodejs - node.js

Question

I was trying to create an endpoint in node.js, more specifically in express
but I am not sure why I can't access userService variable when requesting from a client.
I've gotten Cannot read property 'userService' of undefined, but when i move ServicesFactory.getInstance().getUserService() inside the signUp function it works?!
I am guessing that node.js garbage collects it due to it's not being used until the user make a request.
export class UserApi implements WebEndpoint {
router: Router
userService = ServicesFactory.getInstance().getUserService()
constructor() {
this.router = Router()
this.router.post('/signup', this.signUp)
}
signUp(req: Request, res: Response): void {
const user: User = req.body
this.userService.signUp(user)
res.send("Successfully registered")
}
}

Answer 1

I found the problem, so basically I am a noob.
consider this example
class a {
constructor() {
this.a1 = 'hello';
}
greet(){
const greeting = `${this.a1} dude!`;
console.log(greeting);
};
}
class b {
b1 = new a();
constructor() {
this.b1.greet.call();
}
}
new b();
Now it wouldn't run, because b class called greet method with a new context, the same with express when you provide a function as a handler on an Express endpoint it will be called with a new set of context (read:this) that's why this.userService in my code above won't work because there is no userService property in the context provided by Express.
The solution is simple. Arrow function.
signUp = (req: Request, res: Response): void => {
const user: User = req.body
this.userService.signUp(user)
res.send("Successfully registered")
}
Now the function will inherit it's class's context.You can refer to this for more detail answer.