Question

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.

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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 1

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:

• create function in javascript with custom prototype
• Can you create functions with custom prototypes in JavaScript?
• Is it possible to create a function with another prototype than Function.prototype?
• Can a JavaScript object have a prototype chain, but also be a function?
• Custom prototype chain for a function
• Correct prototype chain for Function (for some internals)

_{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.}