How to extend Function with ES6 classes?

Question

ES6 allows to extend special objects. So it's possible to inherit from the function. Such object can be called as a function, but how can I implement the logic for such call?

class Smth extends Function {
  constructor (x) {
    // What should be done here
    super();
  }
}

(new Smth(256))() // to get 256 at this call?

---

Any method of class gets reference to the class instance via this. But when it is called as a function, this refers to window. How can I get the reference to the class instance when it is called as a function?

PS: Same question in Russian.

Answer 1

The super call will invoke the Function constructor, which expects a code string. If you want to access your instance data, you could just hardcode it:

class Smth extends Function {
  constructor(x) {
    super("return "+JSON.stringify(x)+";");
  }
}

but that's not really satisfying. We want to use a closure.

Having the returned function be a closure that can access your instance variables is possible, but not easy. The good thing is that you don't have to call super if you don't want to - you still can return arbitrary objects from your ES6 class constructors. In this case, we'd do

class Smth extends Function {
  constructor(x) {
    // refer to `smth` instead of `this`
    function smth() { return x; };
    Object.setPrototypeOf(smth, Smth.prototype);
    return smth;
  }
}

But we can do even better, and abstract this thing out of Smth:

class ExtensibleFunction extends Function {
  constructor(f) {
    return Object.setPrototypeOf(f, new.target.prototype);
  }
}

class Smth extends ExtensibleFunction {
  constructor(x) {
    super(function() { return x; }); // closure
    // console.log(this); // function() { return x; }
    // console.log(this.prototype); // {constructor: …}
  }
}
class Anth extends ExtensibleFunction {
  constructor(x) {
    super(() => { return this.x; }); // arrow function, no prototype object created
    this.x = x;
  }
}
class Evth extends ExtensibleFunction {
  constructor(x) {
    super(function f() { return f.x; }); // named function
    this.x = x;
  }
}

Admittedly, this creates an additional level of indirection in the inheritance chain, but that's not necessarily a bad thing (you can extend it instead of the native Function). If you want to avoid it, use

function ExtensibleFunction(f) {
  return Object.setPrototypeOf(f, new.target.prototype);
}
ExtensibleFunction.prototype = Function.prototype;

but notice that Smth will not dynamically inherit static Function properties.

Answer 2

Firstly I came to solution with arguments.callee, but it was awful.
I expected it to break in global strict mode, but seems like it works even there.

class Smth extends Function {
  constructor (x) {
    super('return arguments.callee.x');
    this.x = x;
  }
}

(new Smth(90))()

It was a bad way because of using arguments.callee, passing the code as a string and forcing its execution in non-strict mode. But than idea to override apply appeared.

var global = (1,eval)("this");

class Smth extends Function {
  constructor(x) {
    super('return arguments.callee.apply(this, arguments)');
    this.x = x;
  }
  apply(me, [y]) {
    me = me !== global && me || this;
    return me.x + y;
  }
}

And the test, showing I'm able to run this as function in different ways:

var f = new Smth(100);

[
f instanceof Smth,
f(1),
f.call(f, 2),
f.apply(f, [3]),
f.call(null, 4),
f.apply(null, [5]),
Function.prototype.apply.call(f, f, [6]),
Function.prototype.apply.call(f, null, [7]),
f.bind(f)(8),
f.bind(null)(9),
(new Smth(200)).call(new Smth(300), 1),
(new Smth(200)).apply(new Smth(300), [2]),
isNaN(f.apply(window, [1])) === isNaN(f.call(window, 1)),
isNaN(f.apply(window, [1])) === isNaN(Function.prototype.apply.call(f, window, [1])),
] == "true,101,102,103,104,105,106,107,108,109,301,302,true,true"

Version with

super('return arguments.callee.apply(arguments.callee, arguments)');

in fact contains bind functionality:

(new Smth(200)).call(new Smth(300), 1) === 201

Version with

super('return arguments.callee.apply(this===(1,eval)("this") ? null : this, arguments)');
...
me = me || this;

makes call and apply on window inconsistent:

isNaN(f.apply(window, [1])) === isNaN(f.call(window, 1)),
isNaN(f.apply(window, [1])) === isNaN(Function.prototype.apply.call(f, window, [1])),

so the check should be moved into apply:

super('return arguments.callee.apply(this, arguments)');
...
me = me !== global && me || this;

Answer 3

You can wrap the Smth instance in a Proxy with an apply (and maybe construct) trap:

class Smth extends Function {
  constructor (x) {
    super();
    return new Proxy(this, {
      apply: function(target, thisArg, argumentsList) {
        return x;
      }
    });
  }
}
new Smth(256)(); // 256

Answer 4

Update:

Unfortunately this doesn't quite work because it's now returning a function object instead of a class, so it seems this actually can't be done without modifying the prototype. Lame.

---

Basically the problem is there is no way of setting the this value for the Function constructor. The only way to really do this would be to use the .bind method afterwards, however this is not very Class-friendly.

We could do this in a helper base class, however this does does not become available until after the initial super call, so it's a bit tricky.

Working Example:

'use strict';

class ClassFunction extends function() {
    const func = Function.apply(null, arguments);
    let bound;
    return function() {
        if (!bound) {
            bound = arguments[0];
            return;
        }
        return func.apply(bound, arguments);
    }
} {
    constructor(...args) {
        (super(...args))(this);
    }
}

class Smth extends ClassFunction {
    constructor(x) {
        super('return this.x');
        this.x = x;
    }
}

console.log((new Smth(90))());

(Example requires modern browser or node --harmony.)

Basically the base function ClassFunction extends will wrap the Function constructor call with a custom function which is similar to .bind, but allows binding later, on the first call. Then in the ClassFunction constructor itself, it calls the returned function from super which is now the bound function, passing this to finish setting up the custom bind function.

(super(...))(this);

This is all quite a bit complicated, but it does avoid mutating the prototype, which is considered bad-form for optimization reasons and can generate warnings in browser consoles.

Answer 5

I took the advice from Bergi's answer and wrapped it into an NPM module.

var CallableInstance = require('callable-instance');

class ExampleClass extends CallableInstance {
  constructor() {
    // CallableInstance accepts the name of the property to use as the callable
    // method.
    super('instanceMethod');
  }

  instanceMethod() {
    console.log("instanceMethod called!");
  }
}

var test = new ExampleClass();
// Invoke the method normally
test.instanceMethod();
// Call the instance itself, redirects to instanceMethod
test();
// The instance is actually a closure bound to itself and can be used like a
// normal function.
test.apply(null, [ 1, 2, 3 ]);

Answer 6

This is the solution I've worked out that serves all my needs of extending functions and has served me quite well. The benefits of this technique are:

• When extending ExtensibleFunction, the code is idiomatic of extending any ES6 class (no, mucking about with pretend constructors or proxies).
• The prototype chain is retained through all subclasses, and instanceof / .constructor return the expected values.
• .bind() .apply() and .call() all function as expected. This is done by overriding these methods to alter the context of the "inner" function as opposed to the ExtensibleFunction (or it's subclass') instance.
• .bind() returns a new instance of the functions constructor (be it ExtensibleFunction or a subclass). It uses Object.assign() to ensure the properties stored on the bound function are consistent with those of the originating function.
• Closures are honored, and arrow functions continue to maintain the proper context.
• The "inner" function is stored via a Symbol, which can be obfuscated by modules or an IIFE (or any other common technique of privatizing references).

And without further ado, the code:

// The Symbol that becomes the key to the "inner" function 
const EFN_KEY = Symbol('ExtensibleFunctionKey');

// Here it is, the `ExtensibleFunction`!!!
class ExtensibleFunction extends Function {
  // Just pass in your function. 
  constructor (fn) {
    // This essentially calls Function() making this function look like:
    // `function (EFN_KEY, ...args) { return this[EFN_KEY](...args); }`
    // `EFN_KEY` is passed in because this function will escape the closure
    super('EFN_KEY, ...args','return this[EFN_KEY](...args)');
    // Create a new function from `this` that binds to `this` as the context
    // and `EFN_KEY` as the first argument.
    let ret = Function.prototype.bind.apply(this, [this, EFN_KEY]);
    // For both the original and bound funcitons, we need to set the `[EFN_KEY]`
    // property to the "inner" function. This is done with a getter to avoid
    // potential overwrites/enumeration
    Object.defineProperty(this, EFN_KEY, {get: ()=>fn});
    Object.defineProperty(ret, EFN_KEY, {get: ()=>fn});
    // Return the bound function
    return ret;
  }

  // We'll make `bind()` work just like it does normally
  bind (...args) {
    // We don't want to bind `this` because `this` doesn't have the execution context
    // It's the "inner" function that has the execution context.
    let fn = this[EFN_KEY].bind(...args);
    // Now we want to return a new instance of `this.constructor` with the newly bound
    // "inner" function. We also use `Object.assign` so the instance properties of `this`
    // are copied to the bound function.
    return Object.assign(new this.constructor(fn), this);
  }

  // Pretty much the same as `bind()`
  apply (...args) {
    // Self explanatory
    return this[EFN_KEY].apply(...args);
  }

  // Definitely the same as `apply()`
  call (...args) {
    return this[EFN_KEY].call(...args);
  }
}

/**
 * Below is just a bunch of code that tests many scenarios.
 * If you run this snippet and check your console (provided all ES6 features
 * and console.table are available in your browser [Chrome, Firefox?, Edge?])
 * you should get a fancy printout of the test results.
 */

// Just a couple constants so I don't have to type my strings out twice (or thrice).
const CONSTRUCTED_PROPERTY_VALUE = `Hi, I'm a property set during construction`;
const ADDITIONAL_PROPERTY_VALUE = `Hi, I'm a property added after construction`;

// Lets extend our `ExtensibleFunction` into an `ExtendedFunction`
class ExtendedFunction extends ExtensibleFunction {
  constructor (fn, ...args) {
    // Just use `super()` like any other class
    // You don't need to pass ...args here, but if you used them
    // in the super class, you might want to.
    super(fn, ...args);
    // Just use `this` like any other class. No more messing with fake return values!
    let [constructedPropertyValue, ...rest] = args;
    this.constructedProperty = constructedPropertyValue;
  }
}

// An instance of the extended function that can test both context and arguments
// It would work with arrow functions as well, but that would make testing `this` impossible.
// We pass in CONSTRUCTED_PROPERTY_VALUE just to prove that arguments can be passed
// into the constructor and used as normal
let fn = new ExtendedFunction(function (x) {
  // Add `this.y` to `x`
  // If either value isn't a number, coax it to one, else it's `0`
  return (this.y>>0) + (x>>0)
}, CONSTRUCTED_PROPERTY_VALUE);

// Add an additional property outside of the constructor
// to see if it works as expected
fn.additionalProperty = ADDITIONAL_PROPERTY_VALUE;

// Queue up my tests in a handy array of functions
// All of these should return true if it works
let tests = [
  ()=> fn instanceof Function, // true
  ()=> fn instanceof ExtensibleFunction, // true
  ()=> fn instanceof ExtendedFunction, // true
  ()=> fn.bind() instanceof Function, // true
  ()=> fn.bind() instanceof ExtensibleFunction, // true
  ()=> fn.bind() instanceof ExtendedFunction, // true
  ()=> fn.constructedProperty == CONSTRUCTED_PROPERTY_VALUE, // true
  ()=> fn.additionalProperty == ADDITIONAL_PROPERTY_VALUE, // true
  ()=> fn.constructor == ExtendedFunction, // true
  ()=> fn.constructedProperty == fn.bind().constructedProperty, // true
  ()=> fn.additionalProperty == fn.bind().additionalProperty, // true
  ()=> fn() == 0, // true
  ()=> fn(10) == 10, // true
  ()=> fn.apply({y:10}, [10]) == 20, // true
  ()=> fn.call({y:10}, 20) == 30, // true
  ()=> fn.bind({y:30})(10) == 40, // true
];

// Turn the tests / results into a printable object
let table = tests.map((test)=>(
  {test: test+'', result: test()}
));

// Print the test and result in a fancy table in the console.
// F12 much?
console.table(table);

Edit

Since I was in the mood, I figured I'd publish a package for this on npm.

Answer 7

This is an approach to creating callable objects that correctly reference their object members, and maintain correct inheritance, without messing with prototypes.

Simply:

class ExFunc extends Function {
  constructor() {
    super('...args', 'return this.__self__.__call__(...args)')
    var self = this.bind(this)
    this.__self__ = self
    return self
  }

  // Example `__call__` method.
  __call__(a, b, c) {
    return [a, b, c];
  }
}

Extend this class and add a __call__ method, more below...

An explanation in code and comments:

// This is an approach to creating callable objects
// that correctly reference their own object and object members,
// without messing with prototypes.

// A Class that extends Function so we can create
// objects that also behave like functions, i.e. callable objects.
class ExFunc extends Function {
  constructor() {
    super('...args', 'return this.__self__.__call__(...args)');
    // Here we create a function dynamically using `super`, which calls
    // the `Function` constructor which we are inheriting from. Our aim is to create
    // a `Function` object that, when called, will pass the call along to an internal
    // method `__call__`, to appear as though the object is callable. Our problem is
    // that the code inside our function can't find the `__call__` method, because it
    // has no reference to itself, the `this` object we just created.
    // The `this` reference inside a function is called its context. We need to give
    // our new `Function` object a `this` context of itself, so that it can access
    // the `__call__` method and any other properties/methods attached to it.
    // We can do this with `bind`:
    var self = this.bind(this);
    // We've wrapped our function object `this` in a bound function object, that
    // provides a fixed context to the function, in this case itself.
    this.__self__ = self;
    // Now we have a new wrinkle, our function has a context of our `this` object but
    // we are going to return the bound function from our constructor instead of the
    // original `this`, so that it is callable. But the bound function is a wrapper
    // around our original `this`, so anything we add to it won't be seen by the
    // code running inside our function. An easy fix is to add a reference to the
    // new `this` stored in `self` to the old `this` as `__self__`. Now our functions
    // context can find the bound version of itself by following `this.__self__`.
    self.person = 'Hank'
    return self;
  }
  
  // An example property to demonstrate member access.
  get venture() {
    return this.person;
  }
  
  // Override this method in subclasses of ExFunc to take whatever arguments
  // you want and perform whatever logic you like. It will be called whenever
  // you use the obj as a function.
  __call__(a, b, c) {
    return [this.venture, a, b, c];
  }
}

// A subclass of ExFunc with an overridden __call__ method.
class DaFunc extends ExFunc {
  constructor() {
    super()
    this.a = 'a1'
    this.b = 'b2'
    this.person = 'Dean'
  }

  ab() {
    return this.a + this.b
  }
  
  __call__(ans) {
    return [this.ab(), this.venture, ans];
  }
}

// Create objects from ExFunc and its subclass.
var callable1 = new ExFunc();
var callable2 = new DaFunc();

// Inheritance is correctly maintained.
console.log('\nInheritance maintained:');
console.log(callable2 instanceof Function);  // true
console.log(callable2 instanceof ExFunc);  // true
console.log(callable2 instanceof DaFunc);  // true

// Test ExFunc and its subclass objects by calling them like functions.
console.log('\nCallable objects:');
console.log( callable1(1, 2, 3) );  // [ 'Hank', 1, 2, 3 ]
console.log( callable2(42) );  // [ 'a1b2', Dean', 42 ]

// Test property and method access
console.log(callable2.a, callable2.b, callable2.ab())

View on repl.it

Further explanation of bind:

function.bind() works much like function.call(), and they share a similar method signature:

fn.call(this, arg1, arg2, arg3, ...); more on mdn

fn.bind(this, arg1, arg2, arg3, ...); more on mdn

In both the first argument redefines the this context inside the function. Additional arguments can also be bound to a value. But where call immediately calls the function with the bound values, bind returns an "exotic" function object that transparently wraps the original, with this and any arguments preset.

So when you define a function then bind some of its arguments:

var foo = function(a, b) {
  console.log(this);
  return a * b;
}

foo = foo.bind(['hello'], 2);

You call the bound function with only the remaining arguments, its context is preset, in this case to ['hello'].

// We pass in arg `b` only because arg `a` is already set.
foo(2);  // returns 4, logs `['hello']`

Answer 8

There is a simple solution which takes advantage of JavaScript's functional capabilities: Pass the "logic" as a function-argument to the constructor of your class, assign the methods of that class to that function, then return that function from the constructor as the result:

class Funk
{
    constructor (f)
    { let proto       = Funk.prototype;
      let methodNames = Object.getOwnPropertyNames (proto);
      methodNames.map (k => f[k] = this[k]);
      return f;
    }

    methodX () {return 3}
}

let myFunk  = new Funk (x => x + 1);
let two     = myFunk(1);         // == 2
let three   = myFunk.methodX();  // == 3

The above was tested on Node.js 8.

A shortcoming of the example above is it does not support methods inherited from the superclass-chain. To support that, simply replace "Object . getOwnPropertyNames(...)" with something that returns also the names of inherited methods. How to do that I believe is explained in some other question-answer on Stack Overflow :-). BTW. It would be nice if ES7 added a method to produce inherited methods' names as well ;-).

If you need to support inherited methods one possibility is adding a static method to the above class which returns all inherited and local method names. Then call that from the constructor. If you then extend that class Funk, you get that static method inherited along as well.

Answer 9

Generalizing Oriol's answer:

class Smth extends Function {
  constructor(x) {
    super();

    this.x = x;

    return new Proxy(this, {
      apply: (target, that, args) => target.__call__(...args)
    });
  }

  __call__(v) {
    return this.x * v;
  }
}

Answer 10

A little late but let me leave this here.

Recently I had to discover a way to subclass Function in order to turn normal functions into Threadable / Promisified functions without messing with the Function.prototype. I think this particular necessity forms a very reasonable basis to this question on how and why one can use class abstraction to extend Function.

So the idea is, we create a Threadable class of which the member functions are threadable. By this I mean, any normal function can easily be made Threadable and when spawned runs on a seperate thread and gives us a promise to be resolved or rejected depending on the outcome of the worker operation. However you should still be able to invoke it syncronously if need be.

class Threadable extends Function {
  
  // just use super() to wrap the given f function with a transparent function layer
  constructor(f){
    super("...as",`return ${f.toString()}.apply(this,as)`);
  }
  
  // spawn is the only method of the Threadable class.
  // Returns a promise and runs callee function on a separate thread.
  spawn(...as){
    var code = `self.onmessage = m => self.postMessage((${this.toString()}).apply(self,m.data));`,
        blob = new Blob([code], {type: "text/javascript"}),
        wrkr = new Worker(window.URL.createObjectURL(blob));
    return new Promise( (v,x) => ( wrkr.onmessage = m => (v(m.data), wrkr.terminate())
                                 , wrkr.onerror   = e => (x(e.message), wrkr.terminate())
                                 , wrkr.postMessage(as)
                                 )
                      );
  }
}

function add(...ns) {
    return ns.reduce((a,b) => a+b);
}

var addT = new Threadable(add);

addT.spawn(1,2,3,4)
    .then(m => console.log(`Promisified thread returned ${m}`));

console.log(`Synchronous invocation of addT returned ${addT(1,2,3,4,5)}`);

Answer 11

Having read this article and all the answers here, I finally found a satisfying answer in an older thread.

Here's an example:

class Hey {
  constructor() {
    function hey() {
      return "hey";
    }
    
    this.value = "yo";

        Object.assign(hey, this);
        Object.setPrototypeOf(hey, Object.getPrototypeOf(this));

        return hey;
  }
  
  yo() {
    return this.value;
  }
}

const hey = new Hey();

console.log(hey());              // it's callable 
console.log(hey.yo());           // methods are correctly bound to `this` 

console.log(hey instanceof Hey); // it type-checks 

Inheritance works too:

class HeyHey extends Hey {
  constructor() {
    super();
  }
  
  yoyo() {
    return this.value + "!";
  }
}

const lol = new HeyHey();

console.log(lol());                 // it's callable 
console.log(lol.yo());              // inherited methods are correctly bound to `this` 
console.log(lol.yoyo());            // new methods are correctly bound to `this` as well 

console.log(lol instanceof Hey);    // it type-checks for the super class 
console.log(lol instanceof HeyHey); // it type-checks for the derived class 

You can run the example here to see for yourself.

This approach:

• correctly binds methods to this
• is type-safe (can be type-checked using instanceof)
• correctly supports inheritance (correctly binds and type-checks derived classes)
• doesn't rely on any features newer than class - getPrototypeOf and setPrototypeOf were widely available some years before class
• doesn't rely on the Function constructor (avoids parsing source code at run-time)
• doesn't rely on Proxy (which isn't great for performance)

All in all, this approach is definitely simpler and easier to implement, and it should perform better as well.

(In theory - please feel free to benchmark this and post your results.)

Answer 12

Came up with a solution that works without using Object.setPrototypeOf since MDN has big red warning signs around that. Can run the example JSFiddle here. One limitation I can't figure out is how to get access to the this context at call time of the produced function within the arbitrary execute.

class ExtendedFunction extends Function {

    // Arbitrary private properties
  #foo
  #baz() { return this.#foo + 'qux' }
  
  // The thing that happens when you call your extended function
  // context is optional if you want access to the `this` 
  // provides to your extended function at call time
  #execute() {
    // Arbitrary code that can call anything in closure here
    return this.#baz()
  }
  
  constructor(a) {
    // Set `this` to simple wrapper function
    // that takes another function and returns its value
    // Use super we get an instance of Function and ExtendedFucntion
    super('execute', 'return execute()')
    this.#foo = a
    // Bind our arbitrary function to the current `this`
    // allowing it to access private properties even when passed around
    const boundExecute = this.#execute.bind(this)
    // Bind the simple wrapper and the boundExecute together and return that
    // When called our extended function will do whatever is in #execute
    var self = this.bind(null, boundExecute)
    return self
  }

}

const a = new ExtendedFunction(256)
console.log(a instanceof Function) // true
console.log(a instanceof ExtendedFunction) // true
console.log(a()) // 256qux