"TypeError: Illegal invocation" when trying to proxy object in JS, for debugging?

Question

As part of some code refactoring, I am wanting to trace the sequence of calls to an API, since we seem to have broken something in the process. For this reason we are looking to proxy the API calls, to log what is called, using the Javascript Proxy object, but we are getting "TypeError: Illegal invocation" when we try.

The code (based on MDN docs):

function createProxy (glObj) {
  const handler = {
    get (target, prop, receiver) {
      console.log('FN', prop);
      const x = Reflect.get(target, prop, receiver);
      return x;
    }
  };

  return new Proxy(glObj, handler);
}

And how it is being used:

function webGlInit () {
  const elementId = 'canvas';
  const canvas = document.querySelector(`#${elementId}`); // $('#canvas')[0];

  canvas.width = window.innerWidth;
  canvas.height = window.innerHeight;

  const glOptions = {
    // antialias: false,
    alpha: false,
    premultipliedAlpha: false
  };

  const gl = canvas.getContext('webgl', glOptions) || canvas.getContext('experimental-webgl', glOptions);

  const proxyGl = createProxy(gl);
  // the line where "TypeError: Illegal invocation" is happening
  proxyGl.viewport(0, 0, gl.drawingBufferWidth, gl.drawingBufferHeight);

Any ideas on what we are doing wrong?

See Why is Proxy to a Map object in ES2015 not working, Why is Set Incompatible with Proxy?, How do I apply get Proxy on window.location in Javascript?, How to fix TypeError: Illegal Invocation on an XMLHttpRequest wrapped in an ES6 Proxy — Bergi
– Bergi, Commented Sep 21, 2023 at 18:49

Unmitigated · Accepted Answer · 2023-09-21 00:54:00Z

Unfortunately (I would consider that a flaw since Proxy/Reflect should be transparent) you should bind methods to the target:

function createProxy (glObj) {
  const handler = {
    get (target, prop, receiver) {
      if(typeof target[prop] === 'function'){
         return target[prop].bind(target);
      }      

      const x = Reflect.get(target, prop, receiver);
      return x;
    }
  };

  return new Proxy(glObj, handler);
}

That would cover most cases, but you cannot rebind a function. If you need binding (this also demonstrates how native objects are good with this (RegExp Iterator in this case)):

function createProxy (glObj) {
  const handler = {
    get (target, prop, receiver) {
      if(typeof target[prop] === 'function'){
         return function(){
            return target[prop].apply(this === receiver ? target : this, arguments);
         };
      }      

      const x = Reflect.get(target, prop, receiver);
      return x;
    }
  };

  return new Proxy(glObj, handler);
}

const a = createProxy({name:'a', printName(){ console.log(this.name) }});
const b = {name: 'b'};

a.printName();
a.printName.bind(b)();
a.printName.call(b);

const iterator = createProxy('----'.matchAll(/-/g));
for(const c of iterator){
  console.log(...c);
}

Here we check if the call was made on the proxy we bind to the target, otherwise we use bound this.

This will also remove name, length props of a method, but they are used rarely, if you need them I guess even a more advanced solution could be figured out.

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