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!"

// 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!"