Can't set "apply" trap to Proxy object

Question

I created a Proxy object with an "apply" trap:

var target = {},
    handler = { apply: () => 42 }
    proxy = new Proxy(target, handler);
proxy(); // TypeError: proxy is not a function

Therefore, the Proxy object should be callable. However, it doesn't work.

Why?

Answer 1

According to the definition of the [[Call]] internal method of Proxy objects it should work:

• Let trap be GetMethod(handler, "apply").
• Return Call(trap, handler, «target, thisArgument, CreateArrayFromList(argumentsList)»).

However, there is a problem: not all Proxy objects have the [[Call]] method:

A Proxy exotic object only has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.

Therefore, the target must be a function object:

var target = () => {},
    handler = { apply: () => 42 }
    proxy = new Proxy(target, handler);
proxy(); // 42

Note that I defined target using an arrow function in order to create a function object which is not a constructor function. This way the Proxy object can be called but not instantiated.

If you want to add a "construct" trap too, the target must have a [[Construct]] method, so define it with a function declaration or function expression.

Answer 2

I came here from Google searching for 'callable proxy'.
While the existing answer works, there is another alternative that may be 'cleaner' for some purposes:

class Callable extends Function {
  constructor() {
    super()    
    return new Proxy(this, {
      apply: (target, thisArg, args) => target._call(...args)
    })
  }
  
  _call(...args) {
    console.log(this, args)
  }
}

You can define all traps in proxy as usual.
All credits to arccoza and his gist here.