How to call an object method by Proxy in Javascript?

Question

I have the class:

class Course {
  constructor(name) {
    this._name = name;
  }

  getName() {
    return this._name;
  }
}

I want to create a proxy around class Course, that will return only non-private fields and methods:

const handlers = {
  get: (target, prop, reciever) => {
    if (prop in reciever && prop[0] !== '_' ) {
      if (typeof reciever[prop] === 'function') {
        return reciever[prop].apply(reciever);
      } else {
        return reciever[prop];
      }
    } else {
      throw new Error('problem');
    }
  }
}

const protect = (obj) => {
  return new Proxy(obj, handlers);
}

But when i call object method:

const protectedCourse = protect(new Course('Test'));
console.log(protectedCourse.getName()); // The expected result is - "Test"

I got the error:

  if (prop in reciever && prop[0] !== '_' ) {
    ^

RangeError: Maximum call stack size exceeded
    at Object.get (file:///usr/src/app/protect.js:37:5)
    at Object.get (file:///usr/src/app/protect.js:38:26)
    at Object.get (file:///usr/src/app/protect.js:38:26)
    at Object.get (file:///usr/src/app/protect.js:38:26)
    at Object.get (file:///usr/src/app/protect.js:38:26)
    at Object.get (file:///usr/src/app/protect.js:38:26)
    at Object.get (file:///usr/src/app/protect.js:38:26)
    at Object.get (file:///usr/src/app/protect.js:38:26)
    at Object.get (file:///usr/src/app/protect.js:38:26)
    at Object.get (file:///usr/src/app/protect.js:38:26)

How to call object method without infinity recursion in the handler of Proxy?

What's the actual error you get?

Mike 'Pomax' Kamermans
– Mike 'Pomax' Kamermans

2021-11-15 17:53:06 +00:00
Commented Nov 15, 2021 at 17:53 — Mike 'Pomax' Kamermans
– Mike 'Pomax' Kamermans, Commented Nov 15, 2021 at 17:53
Maximum call stack size exceeded

Yuri Molodyko
– Yuri Molodyko

2021-11-15 17:54:24 +00:00
Commented Nov 15, 2021 at 17:54 — Yuri Molodyko
– Yuri Molodyko, Commented Nov 15, 2021 at 17:54

Unmitigated · Accepted Answer · 2021-11-15 17:58:44Z

You should access the property on target, which is the original object, instead of receiver which is the Proxy. Additionally, use bind instead of apply or call to return a function with the this value set.

Note that what you are trying to achieve with a Proxy can be done with private class fields.

class Course {
  constructor(name) {
    this._name = name;
  }

  getName() {
    return this._name;
  }
}

const handlers = {
  get: (target, prop, receiver) => {
    if (prop in target && prop[0] !== '_') {
      if (typeof target[prop] === 'function') {
        return target[prop].bind(target);
      } else {
        return target[prop];
      }
    } else {
      throw new Error('problem');
    }
  }
}

const protect = (obj) => {
  return new Proxy(obj, handlers);
}

const protectedCourse = protect(new Course('Test'));
console.log(protectedCourse.getName());

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