Why is `this` undefined in a constructor defined Proxy method for a derived class?

Question

I attempting to overwrite/proxy a function from a library class. I've done this by extending the class and defining a Proxy object over the method.

Alas, like in this question, the this object in the derived class is undefined. I can't seem to get those solutions working in my case. Perhaps due to use of Proxy.

See example below:

class Structure {
    constructor (name){
        this.noDoors = 0
        this.name = name
    }
    async addDoors(noDoorsToAdd){
        this.noDoors += noDoorsToAdd
        console.log(`${noDoorsToAdd} doors added to ${this.name}`)
        return new Promise(r=>r())
    }
}

const Building = class Building extends Structure {
    constructor(conditionfunc,args){
        super(args)     

        this.addDoors = new Proxy(this.addDoors, {
            apply(target, thisArg, argumentsList){
                //console.log("apply:",target, thisArg, argumentsList,conditionfunc(argumentsList[0]))
                if (conditionfunc(argumentsList[0])){
                    console.log(`Correct password: About to add doors to ${thisArg.name}`)
                    target(argumentsList[1]);
                }
                else {
                    console.log(`Incorrect password: Doors not added to ${thisArg.name}`)
                }
            }
        })
    }
}


/******** Main Script *********/

let conditionfunc = (password) => {
    if (password == '123456') { return true } else {return false }
}

 myStructure = new Structure('Ty Bach')
 myBuilding = new Building(conditionfunc,'Ty Mawr')

 ;(async()=>{
  await myStructure.addDoors(1)
    await myBuilding.addDoors('wrongpassword',7)
    await myBuilding.addDoors('123456',4)
  
  console.log(`${myStructure.name} has ${myStructure.noDoors} doors!`)
    console.log(`${myBuilding.name} has ${myBuilding.noDoors} doors!`)
})();

Here I expect the myBuilding object to end up with 4 doors.

The error messages on my console are:

1 doors added to Ty Bach
Doors not added to Ty Mawr
About to add doors to Ty Mawr
Ty Bach has 1 doors!
Ty Mawr has 0 doors!
(node:3732) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'noDoors' of undefined
    at addDoors (MYPATH\proxytoy\test.js:11:3)
    at Object.apply (MYPATH\proxytoy\test.js:26:6)
    at MYPATH\proxytoy\test.js:51:23
    at processTicksAndRejections (internal/process/task_queues.js:95:5)
(Use `node --trace-warnings ...` to show where the warning was created)
(node:3732) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:3732) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Why is the this undefined in the derived class' object?

How might I fix this, or achieve the same through some other way?

Answer 1

You are calling the original function without assigning this.

Use target.call(thisArg, argumentsList[1]) instead

class Structure {
  constructor(name) {
    this.noDoors = 0
    this.name = name
  }
  async addDoors(noDoorsToAdd) {
    this.noDoors += noDoorsToAdd
    console.log(`${noDoorsToAdd} doors added to ${this.name}`)
    return new Promise(r => r())
  }
}

const Building = class Building extends Structure {
  constructor(conditionfunc, args) {
    super(args)

    this.addDoors = new Proxy(this.addDoors, {
      apply(target, thisArg, argumentsList) {
        //console.log("apply:",target, thisArg, argumentsList,conditionfunc(argumentsList[0]))
        if (conditionfunc(argumentsList[0])) {
          console.log(`Correct password: About to add doors to ${thisArg.name}`)
          target.call(thisArg, argumentsList[1]);
        } else {
          console.log(`Incorrect password: Doors not added to ${thisArg.name}`)
        }
      }
    })
  }
}


/******** Main Script *********/

let conditionfunc = (password) => {
  if (password == '123456') {
    return true
  } else {
    return false
  }
}

myStructure = new Structure('Ty Bach')
myBuilding = new Building(conditionfunc, 'Ty Mawr')

;
(async() => {
  await myStructure.addDoors(1)
  await myBuilding.addDoors('wrongpassword', 7)
  await myBuilding.addDoors('123456', 4)

  console.log(`${myStructure.name} has ${myStructure.noDoors} doors!`)
  console.log(`${myBuilding.name} has ${myBuilding.noDoors} doors!`)
})();