Class member inheritance in nodejs + expressjs

Question

I don't quite understand how class inheritance and context is working in nodejs + expressjs.

I have 3 files :

api.js

class API {
  constructor () {
    this._id = Math.random()
    console.log("Instantiated id:" + this._id)
  }

  list (req, res, next) {
    console.log("Instantiated id:" + this._id)
  }
}
module.exports = API

user.js

const API = require('./api.js')

class UserAPI extends API {

  constructor () {
    super()
  }
}
module.exports = UserAPI

route.js

var UserAPI = require('./user.js')
var user = new UserAPI()
router.get('/', user.list);

What I would like to see is the ID when starting then the same ID each time I do a GET request. But when I do a GET request I have : ERROR: TypeError: Cannot read property '_id' of undefined

How can I have access to member in the Base class when deriving it ?

Thanks !

Classes are syntactic sugar for JS's prototypal inheritance. I your case this no longer comes from the class when you pass the function to router.get. Convert your list to an arrow function and it will work — Wainage
– Wainage, Commented Oct 11, 2017 at 12:13
When I change to router.get('/', user.list.bind(this)); I get Instantiated id:undefined — Thibaud
– Thibaud, Commented Oct 11, 2017 at 12:24
id:undefined? You don't even have an id. Check for typo's — Wainage
– Wainage, Commented Oct 11, 2017 at 12:31
It's just the line of text in the console.log: console.log("Instantiated id:" + this._id) — Thibaud
– Thibaud, Commented Oct 11, 2017 at 12:33

Patrick Hund · Accepted Answer · 2017-10-11 12:56:58Z

2

The problem is not class inheritance, but simply the way you use the list method. You are passing it directly to router.get, that won't work, because you are only passing a reference to the function, which then doesn't know the context of the instance of UserAPI.

You can fix this by passing an anonymous function to router.get that then executes user.list, like this:

route.js

var UserAPI = require('./user.js')
var user = new UserAPI()
router.get('/', (req, res, next) => user.list(req, res, next));

answered Oct 11, 2017 at 12:56

Patrick Hund

20.4k12 gold badges71 silver badges97 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Burawi · Accepted Answer · 2018-01-16 20:59:41Z

0

Making API inheritance with express is impossible. CheckZinkyJS, it makes exactly what you want to do.

answered Jan 16, 2018 at 20:59

Burawi

4715 silver badges16 bronze badges

Collectives™ on Stack Overflow

Class member inheritance in nodejs + expressjs

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related