Implement an interfaces with multiple inheritance in a Nodejs class

Question

I tried the following way to create an interface and implement it.

class AInterface {
  constructor () {
    if (!this.methodA) {
      throw new Error('class should implement a methodA() method')
    } else if (!this.methodB) {
      throw new Error('class should implement a methodB() method')
    }
  }
}
export default AInterface

implemented that in a class by extending it. (Note that I have used ts-mixer to have multiple inheritance.

import AInterface from './AInterface'
import { Mixin } from 'ts-mixer'

class ClassA extends Mixin(AnotherClass, AInterface) {

  constructor () {
    super()
  }

  methodA () {
    return 'test'
  }

  methodB () {
    return 'test'
  }

}
export default ClassA

This will throw the error class should implement a methodA() method. Which means the check I do in the interface fails if (!this.methodA). This works fine when I remove the Mixin and extend only the interface. (class ClassA extends AInterface)

Is there a better way to do this or how can I fix this?

Node version - 14

Dharman · Accepted Answer · 2021-12-22 12:07:20Z

1

The problem appears to be that the constructor of AInterface does not get the right this. So it does not see the methodA or methodB.

The workaround is to avoid doing that check in the constructor.

import AInterface from "./AInterface.mjs";
import { Mixin, settings } from "ts-mixer";

settings.initFunction = "init";

class ClassA extends Mixin(AnotherClass, AInterface) {}

AInterface.js

class AInterface {
  init () {
    if (!this.methodA) {
      throw new Error('class should implement a methodA() method')
    } else if (!this.methodB) {
      throw new Error('class should implement a methodB() method')
    }
  }
}
export default AInterface

Notice the use of init() method above.

edited Dec 22, 2021 at 12:07

Dharman♦

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answered Dec 22, 2021 at 12:02

Nimeshka Srimal

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1 Comment

Bergi Over a year ago

If the constructor does not get the right this value, that sounds very much like a bug in that ts-mixer library?

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