compose.js

// compose function
String.prototype.ᐅ = function(f) {
  return f(this)
}

function doubleSay (str) {
    return str + ", " + str;
}
function capitalize (str) {
    return str[0].toUpperCase() + str.substring(1);
}
function exclaim (str) {
    return str + '!';
}

let result = "hello"
  .ᐅ (doubleSay)
  .ᐅ (capitalize)
  .ᐅ (exclaim)

result //=> "Hello, hello!"

Object.prototype.ᐅ = function(f) {
  return f(this)
}

Makes it available everywhere! Numbers, arrays, strings…

const result = ['hello', (3).ᐅ(doubleSay)]
  .ᐅ (exclaim)
  .ᐅ (capitalize)

result // -> "Hello,3, 3!"

Use Object.defineProperty Luke. ✨

// Compose function everywhere
Object.prototype.$$ = function (f) { return f(this) }

const doubleSay = str => str + ", " + str
const capitalize = str => str[0].toUpperCase() + str.substring(1)
const exclaim = str => str + '!'

let result = "hello"
    .$$(doubleSay)
    .$$(capitalize)
    .$$(exclaim)

result //=> "Hello, hello!"

@steida But neither is working for string!

Object.defineProperty(Object, '$$', {get: function (f) { return f(this) }})
Object.defineProperty(Object, '$$', function (f) { return f(this) })

I prefer the ᐅ symbol because it's look like the |> symbol used in most functional programming languages. But it's not a valid identifier in JavaScript.

Function.prototype.ᐅ = function(f) {
  const self = this
  return function(...args) {
    return f(self(...args))
  }
}

const doubleSay = str => str + ", " + str
const capitalize = str => str[0].toUpperCase() + str.substring(1)
const exclaim = str => str + '!'

const program =
  doubleSay
  .ᐅ (capitalize)
  .ᐅ (exclaim)

program('hello')  //=> "Hello, hello!"

// compose function
const ƒ = (_, ...inputs) => inputs.reduce((res, fn) => fn(res))

const doubleSay = str => str + ", " + str
const capitalize = str => str[0].toUpperCase() + str.substring(1)
const exclaim = n => str => str + '!'.repeat(n)

let input = "hello"

let result = ƒ `
${input}
  |> ${doubleSay}
  |> ${capitalize}
  |> ${exclaim(3)}
`

result //=> "Hello, hello!!!"

I'd use ø because it's not used, and available in english and french.
Also it reminds of the mathematical composition operator (which is just o).

@rickmed has a point here :D

@rickmed we could also add some arrow functions to get rid of the const self = this. Although it might be less readable.

Function.prototype.ᐅ = function(f) {
  return (...args) => f(this(...args))
}

const doubleSay = str => str + ", " + str
const capitalize = str => str[0].toUpperCase() + str.substring(1)
const exclaim = str => str + '!'

const program =
  doubleSay
  .ᐅ (capitalize)
  .ᐅ (exclaim)

program('hello')  //=> "Hello, hello!"