Object.create(Function.prototype) => create Function which inherits properties [duplicate]

Question

I am looking for a way to construct a function that has some inheritance.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/prototype

What I am looking for is an object that is a function that can be instantiated in a program, but that has new properties attached to it's prototype.

So, what we shouldn't do is this:

Function.prototype.x = function(){

};

Function.prototype.y = function(){

};

so what I want to do is something like:

const proto = Object.create(Function.prototype);

proto.x = function(){};

proto.y = function(){};

however, at some point later in the program, I would like to actually define the function body for proto, so that I can actually call proto like a function:

proto();

is there a way to do this? The above definitely does not work :)

This is a little closer, and the error message is telling:

function F(){

}

F.prototype = Function.prototype;

var f = new F();

f.apply(null);

Run the above and we get:

TypeError: Function.prototype.apply was called on [object Object], which is a object and not a function at Object. (/Users/amills/WebstormProjects/oresoftware/suman/exp7.js:15:3) at Module._compile (module.js:541:32) at Object.Module._extensions..js (module.js:550:10) at Module.load (module.js:458:32) at tryModuleLoad (module.js:417:12) at Function.Module._load (module.js:409:3) at Function.Module.runMain (module.js:575:10) at startup (node.js:160:18) at node.js:449:3

I've tried this, the further you go down this road, the more hacky it gets. What do you want to achieve with that? Why does the instance have to be a function? why can't you simply attach the function to the instance? — Thomas
– Thomas, Commented Jun 4, 2016 at 0:31
I believe you, even given the little amount of experimentation that I have done - I am creating a library/API. Inheritance just means more performance, because prototypes are shared objects - what I want is an object that is a function but whose prototype is shared, but is not the function prototype :) probably not going to happen :( — Alexander Mills
– Alexander Mills, Commented Jun 4, 2016 at 0:35
In compatible engines, you can define a class that extends Function (support). — Jonathan Lonowski
– Jonathan Lonowski, Commented Jun 4, 2016 at 0:39
@AlexMills A function is recognized by an internal property, [[Call]]. This isn't inherited from a prototype and Object.create() doesn't have any mechanism to define it. – class works by invoking the Function constructor through super(), which does set [[Call]]. — Jonathan Lonowski
– Jonathan Lonowski, Commented Jun 4, 2016 at 0:49
A possible alternative could be to change the function's prototype after it's been created – var foo = function () {}; Object.setPrototypeOf(foo, proto);. — Jonathan Lonowski
– Jonathan Lonowski, Commented Jun 4, 2016 at 0:53

Alexander Mills · Accepted Answer · 2016-06-13 23:34:00Z

1

The right way to do this is as follows!

step 1

create an Object which inherits from Function.prototype

const obj = Object.create(Function.prototype);  // inherits many properties, etc

step 2

later in your program declare the function

const f = function(){};

step 3

set the __proto__ of f to obj

Object.setPrototypeOf(f,obj); which is basically equivalent to f.__proto__ = obj;

....this seems to work without any problems, thanks for everyone's help!

edited Jun 13, 2016 at 23:34

answered Jun 4, 2016 at 1:02

Alexander Mills

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4 Comments

Alexander Mills Over a year ago

note: this works, except you will not be able to call f.apply() or f.call() or f.toString(), so that will probably be a problem.

Bergi Over a year ago

Please just only show Object.setPrototypeOf and don't even mention the deprecated __proto__ :-)

Oriol Over a year ago

@AlexMills Then make obj inherit from Function.prototype

Alexander Mills Over a year ago

@Oriol thanks, I tried that, and it worked perfectly, good call

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