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

Answer 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 .