Problem when using class ES6 in ExpressJS/NodeJS

Question

I am using class to implement middleware in ExpressJS but I am getting a problem when calling methods. Firstly, I am running the following code below and then I still received res and req objects from applyMiddleware but in the handleMiddleware method I trying to get req and res but it will lead to the error:

TypeError: Cannot read properties of undefined (reading 'MyMiddleware')

why am i getting this error? And what is the cause of this error? It seems to only happen with classes and not with functions.

my code:

class MyMiddleware {
  constructor() {}
  
  handleMiddleware(req, res) {
     // cannot get req & req
     console.log('Request incoming: ', req);
     console.log('Response outcoming: ', res);
  }

  applyMiddleware(req, res, next) {
     console.log(req, res); // oke, everything is printed

     this.handleMiddleware(req, res);
     next();
  }
}

export default MyMiddleware;

Applying the middleware above in Express:

const app = express();
const middleware = new MyMiddleware();
app.use(middleware.applyMiddleware)

...more code...

Answer 1

The problem is that this is not preserved which causes error when you invoke it, So to resolve this issue you could use an arrow function:

handleMiddleware = (req, res) => {
    //code...
}

Or you can use the .bind(this) method to bind the correct this context when you call the function:

handleMiddleware(req, res) {
console.log('handleMiddleware Request and Response:', req, res);
}

applyMiddleware(req, res, next) {
    console.log(req, res);
    this.handleMiddleware.bind(this)(req, res);
    next();
}