How to add a function to Function.prototype in JavaScript?

Question

I'm trying some basic FP techniques in JavaScript. The compose function takes two functions and give out their Composition function. Now I want to make compose a part of the Function such that I can chain it with ease.

var compose = (func1, func2) => (args => func2(func1(args)));
var square = x => x * x;
var cube = x => x * x * x;
var sixthPower = compose(cube, square);
console.log(sixthPower(2)); // 64
console.log(compose(cube, x => x)(3)); // 27
Function.prototype.compose = function(func) {
  return function(args) {
    func(/* not sure what to write here */)
  }
}
// to make this possible
console.log(square.compose(cube)(3));

Composition Function of functions f1 and f2 is f3 such that f3(x) = f2(f1(x)) — Dinesh Panchananam
– Dinesh Panchananam, Commented Jul 14, 2016 at 11:44

meskobalazs · Accepted Answer · 2016-07-14 11:50:26Z

3

You need to store the original this reference, so you can use it inside the returned function:

var square = x => x * x;
var cube = x => x * x * x;

Function.prototype.compose = function(func) {
  var self = this;
  return function(args) {
    return func(self(args))
  }
}

console.log(square.compose(cube)(3));

edited Jul 14, 2016 at 11:50

answered Jul 14, 2016 at 11:45

meskobalazs

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2 Comments

Dinesh Panchananam Over a year ago

Yes, exactly what I was looking for, Thanks.

grappler Over a year ago

An alternative would be with the lambda syntax and thus not requiring this to be saved to a variable. Function.prototype.compose = function(func) { return (args) => func(this(args)) }

Yizheng Shen · Accepted Answer · 2016-07-15 15:37:04Z

1

Personally I am against your approach because of the following reasons:

Extending on Function prototype chain. The Prototype js framework is a good example indicating such actions is harmful. Here is an article from MDN talking about this in detail.
Now compose is a method instead of a function.

I suggest you check the compose function on Ramda (a js functional library)

edited Jul 15, 2016 at 15:37

answered Jul 14, 2016 at 12:19

Yizheng Shen

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5 Comments

user6445533 Over a year ago

Or use function composition in its original sense: const comp = f => g => x => f(g(x)); comp(cube)(square)(3);

Peter Seliger Over a year ago

@YizhengShen ... Opinion: Function.prototype almost from its very beginning did feature apply and call. Later it even got assigned bind. Compose methods provide far more benefit than the latter. Function.prototype therefore is the perfect place for standard implementations of e.g. curry, compose, before, after, around (... only mentioning a few).

user6445533 Over a year ago

That makes me really sad: "Function.prototype therefore is the perfect place for standard implementations of e.g. curry, compose..."

Yizheng Shen Over a year ago

@PeterSeliger I think it's better to favor composition over inheritance, like this: Object.assign(square, {compose(){...}}. I also find a link from MDN which talks about the extending native prototypes. PS: I'm not sure about this but I think apply, call, and bind is irrelevant here because they are for language problems, and now with ES6 there are spread operator, arrow function for binding lexical scope to help

Yizheng Shen Over a year ago

I mean no offense for my following words. If you feel so, I apologize. I think function composition is closer to functional programming function composition than extending square with a compose function which only have the same name and function as the one in functional programming but lack what's indeed.

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