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Creating Callable Objects in JavaScript

by AdrienDecember 7th, 2019

Too Long; Didn't Read

A callable object is a data structure that behaves as both an object and a function. Callable objects can also be thought of as stateful functions. It’s not built in to the language like Python, but there are several ways to make it work. The main hurdle is giving a function object a reference to itself. We’re going to need to inherit from the Function constructor, which inherits from Object. We want that function to be able to access its own object and call a method, passing on its arguments.

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var obj = new CallableObject(); obj(args);

A callable object is a data structure that behaves as both an object and a function. You can access and assign properties

obj.bar

, call methods

obj.foo()

, but also call the object directly

obj()

, as if it were a function.

The direct call is like calling a method of

obj

which has access to the object’s properties through its

this

context.

If you have experience with Python you’ll recognize this, there is a builtin Python protocol using the

__call__

class method. Any method assigned to

__call__

can be accessed as

obj.__call__()

or as

obj()

Callable Objects can also be thought of as stateful functions. Functions are inherently single instance, stateless procedures. Callable Objects are instantiated, stateful procedures.

In JavaScript almost everything is an object, including functions, so surely we can do this, but how? It’s not built in to the language like Python, but there are several ways to make it work.

The Challenge

Taking inspiration from Python, we want to create a Class / constructor that we can use to create callable objects that redirect their calls to a method named

_call

. We want this redirect so that we can inherit from our Class and easily override and extend the

_call

method with new functionality, without having to worry about the inner workings of the callable object.

To do this, we’re going to need to inherit from the Function constructor, which inherits from Object, and allows us to create both an object and a dynamic function.

Our main hurdle is giving a function object a reference to itself.

In order to have a reference to the

_call

method, the function part of our function object, generated by our Callable class / constructor, must have a reference to itself.

The Solutions

We want to create an extensible

Callable

class that maintains proper and correct inheritance in JavaScript, and allows us to call the objects it constructs as functions, with a reference to themselves, redirecting those calls to an overridable method

_call

The Bind Way

'use strict'

class Callable extends Function {
  constructor() {
    super('...args', 'return this._bound._call(...args)')
    // Or without the spread/rest operator:
    // super('return this._bound._call.apply(this._bound, arguments)')
    this._bound = this.bind(this)

    return this._bound
  }
  
  _call(...args) {
    console.log(this, args)
  }
}

Try it out on repl.it

Because we’re inheriting from Function we can create dynamic functions from strings, using super in our constructor. So the string we pass to

super

will be the body of our function. We want that function to be able to access its own object and call a method

_call

, passing on its arguments. We do this using bind.

The bind method will allow us to set the

this

context of a function to whatever we want, by wrapping that function in a transparent bound function. So we bind the function to itself with

this.bind(this)

Now our callable object has a reference to itself, except the object we return from our constructor, returned by

this.bind

, is a wrapped version of our original object. So all our properties will be attached to that, new, wrapped function object, and our function has a reference to the old object passed to

bind

The easy solution to this is to attach a reference to the new wrapped object on the old object as

_bound

. And the body of our function, in the string passed to

super

, simply calls

_call

using the

this._bound

reference.

Pros

Doesn’t rely on deprecated or modern features.
No need to modify prototypes.

Cons

Requires wrapping the function object in a bound function.

The Callee Way

'use strict'

class Callable extends Function {
  constructor() {
    super('return arguments.callee._call.apply(arguments.callee, arguments)')
    // We can't use the rest operator because of the strict mode rules.
    // But we can use the spread operator instead of apply:
    // super('return arguments.callee._call(...arguments)')
  }
  
  _call(...args) {
    console.log(this, args)
  }
}

Try it out on repl.it

Again we use the

super

call to create a dynamic function, but his time we get our reference to the function itself by taking advantage of another implicit variable inside a function, the arguments object.

The arguments object has a property arguments.callee that is a reference to the called function. We use this reference as the first argument to

apply

which binds the functions

this

context to itself.

So inside the function body, the string passed to super, we only need to call the

_call

method on

arguments.callee

Pros

Very simple.
No need to modify prototypes.

Cons

```
arguments
```
and
```
arguments.callee
```
are unavailable in ‘strict mode’, see MDN for more.
Modern code should avoid
```
arguments
```
for the spread/rest operator.

The Closure & Prototype Way

'use strict'

class Callable extends Function {
  constructor() {
    var closure = function(...args) { return closure._call(...args) }
    // Or without the spread/rest operator:
    // var closure = function() {
    //   return closure._call.apply(closure, arguments)
    // }
    return Object.setPrototypeOf(closure, new.target.prototype)
  }
  
  _call(...args) {
    console.log(this, args)
  }
}

Try it out on repl.it

Here instead of creating a dynamic function with

super

, we discard the function object created by the constructor ( the

this

object ), and replace it with a closure, by returning it instead of

this

from the

constructor

The closure is also a function object, and can reference itself in its body through the closed over

closure

variable. We use the

closure

reference to redirect calls to its method

_call

But we’ve broken the prototype chain by replacing

this

with

closure

, so we reattach the prototype of the constructor to

closure

using Object.setPrototypeOf and new.target (which is a reference to the constructor) to get the prototype.

You can use

this.constructor.prototype

instead of

new.target.prototype

, but then you must first call

super

to create the

this

object, which is wasteful.

Pros

Requires no wrapping of the returned object with a
```
Proxy
```
or
```
bind
```
.

Cons

Requires modifying prototypes.
Modifying prototypes is slow and has other side effects, see MDN.

The Proxy Way

'use strict'

class Callable extends Function {
  constructor() {
    super()    
    return new Proxy(this, {
      apply: (target, thisArg, args) => target._call(...args)
    })
  }
  
  _call(...args) {
    console.log(this, args)
  }
}

Try it out on repl.it

Using Proxy we can intercept calls to a function, using the

apply

trap, and redirect it to another function. The

apply

trap gives our callable a reference to itself as the

target

argument.

So we create a Class that inherits from Function,

Callable

, wrapping the callable objects created in a

Proxy

, trapping any calls made to those objects, and redirecting them to the

_call

method on the object itself, using the

target

reference.

Pros

Simple, native way to intercept calls and redirect them.
No need to modify prototypes.

Cons

Requires wrapping objects created by
```
Callable
```
in a
```
Proxy
```
.
A small performance penalty for using
```
Proxy
```
handlers.

What We Learned

JavaScript is a remarkably flexible language, and you can bend it into many interesting shapes, like Callable Objects. There may well be use cases for Callable Objects, but in general I wouldn’t recommend using them, it’s not idiomatic JavaScript, and might be confusing or unclear to anyone else using your code. This was mostly just an interesting exercise.

References:

https://stackoverflow.com/questions/36871299/how-to-extend-function-with-es6-classes/40878674#40878674