Snowball Snowball - 26 days ago 9
Javascript Question

Define a function with a prototype chain

Suppose I have an object

X
defined as

var X = function () {};
X.prototype.doSomething = function () {};
X.prototype.doSomethingElse = function () {};


Is it possible to construct a function
f
so that
f instanceof X
?
Note that I must also be able to do
f()
without a
TypeError
.




In Mozilla, I can do exactly what I want with
__proto__
:

var f = function () {};
f.__proto__ = new X;


However, that is (1) nonstandard and (2) deprecated. MDN's page for
__proto__
suggests using
Object.getPrototypeOf
instead, but what I'm really looking for is an
Object.setPrototypeOf
(which doesn't exist, though the idea is brought up in this bug report).

A cheap approximation to what I want is

var f = function () {};
jQuery.extend(f, new X);


Unfortunately, this does not make
f instanceof X
true (nor would I
expect it to!).

Answer

No, it is not possible (in a standard way). Every possibility to create a callable object (i.e., a function) will create one inheriting from Function.prototype1; and you can't change the [[prototype]] of an object afterwards2.

See also:

1: OK, ES6 allows us to subclass Function. But that's not as useful as it sounds.
2: Since ES6, you can use Object.setPrototypeOf.