javascript - What does Function.prototype.toMethod() do?

Question

I noticed that the Function.prototype has a toMethod() method in experimental JavaScript, but what does that actually do? And how do I use it?

Answer 1

Update: the toMethod method was experimental only and did not make it into the standard. The home object is essentially static now, the only way to manipulate super is to change the [[prototype]]:

var base = {…}; // as below
var obj = Object.setPrototypeOf({
    foo() { // needs to use method definition syntax
       super.foo();
    }
}, base);
obj.foo();

---

It's very similar to the bind method of function objects. However, instead of creating a new function with a bound this value, it creates a new function with a bound [[HomeObject]], which is the reference that is used for super calls:

[[HomeObject]] (Object): If the function uses super, this is the object whose [[GetPrototypeOf]] provides the object where super property lookups begin.

Consider this example (not using any class syntax):

var base = {
    foo: function() {
         console.log("base foo called on", this);
    }
};
base.foo(); // base foo called on base
var obj = Object.create(base);
obj.foo(); // base foo called on obj

obj.foo = function() {
    super.foo();
};
obj.foo(); // ReferenceError: this method has no home
obj.bar = obj.foo.toMethod(obj);
obj.bar(); // base foo called on obj

obj.baz = function() {
    super();
};
obj.baz(); // ReferenceError: this constructor has no parent class
Reflect.setPrototypeOf(obj.baz, base.foo);
obj.baz(); // base foo called on obj