Illegal invocation while intercepting method calls with javascript proxy

Question

Can someone explain the following output

var handler = {
  get: function(target,key, receiver){
    return new Proxy(Reflect.get(target, key, receiver),handler);
  }, 
  apply: function(target, thisArg, args){
    Reflect.apply(target, thisArg, args);
  }
}

var p = new Proxy(window, handler);
p.alert("Alert"); // Throws illegal invocation error

var alias = p.alert;
alias("Alert") // Even this works which is baffling for me

var p = new Proxy(Reflect.get(window, "alert"), handler);
p("Alert"); // works as expected

The first statement throws an illegal invocation error, whereas the second one doesn't. Both of them look identical to me, and I don't understand how the first statement doesn't have the required this context during Reflect.apply

For @ibrahimmahrir’s answer as it applies to your proxy, see the error produced by alert.call(new Proxy(window, {}), "alert") — Ry-
– Ry- ♦, Commented Aug 21, 2018 at 22:34
When a function is invoked without an explicit context, the context will be window. Explicit context: in both obj.func and func.call(obj) the context will be obj. — ibrahim mahrir
– ibrahim mahrir, Commented Aug 21, 2018 at 22:41
@Ry- Thanks for that wonderful example, it actually answers the question. So passing a proxy object of a target as a context is not the same as passing the target itself as the context, which actually makes sense. — Ayush Goel
– Ayush Goel, Commented Aug 22, 2018 at 16:07

ibrahim mahrir · Accepted Answer · 2018-08-21 22:45:56Z

1

It's just that alert needs window as context, otherwise it throws that error. It has nothing to do with proxies. These two examples throw the same error:

var obj = {};
alert.call(obj, "hello!");           // doesn't work!

and:

var obj = { alert: alert };
obj.alert("hello!");                 // doesn't work!

In your code if you set the context of p.alert to window, it works:

p.alert.call(window, "hello!");      // works!

var handler = {
  get: function(target, key, receiver) {
    return new Proxy(Reflect.get(target, key, receiver), handler);
  },
  apply: function(target, thisArg, args) {
    Reflect.apply(target, thisArg, args);
  }
}

var p = new Proxy(window, handler);
p.alert.call(window, "Alert");

The other two examples from your code work because the context is window.

edited Aug 21, 2018 at 22:45

answered Aug 21, 2018 at 22:31

ibrahim mahrir

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1 Comment

Ayush Goel Over a year ago

Thanks I'll mark this as the answer, but the reason the proxy here becomes relevant is because if I call Reflect.apply inside the get handler itself, this problem no longer occurs, cause now I am calling the method with the target context captured.

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