this is undefined in typescript class

Question

I am trying to understand why "this" can be undefined in classes sometimes, and how to fix it:

import { UserDatabase } from './user.database';

export class UserService {
    private userDatabase = new UserDatabase();
    
    
    async login (request: Request, response: Response) {
        let {email, password} = request.body;
        
        const user = await this.userDatabase.login(email, password);
...

Error log:

(node:3254) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'userDatabase' of undefined

user_632445 · Answer

The usual problem with the this.

You could bind the method to the class context:

export class UserService {
  constructor () {
    this.login = this.login.bind(this);
  }

  ...

}

Read more here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_objects/Function/bind

this is undefined in typescript class

1 Answers