Piping functions with multiple arguments

usenext.js

/**
 * In JavaScript, we can only return one value from a function.
 * 
 * This restricts piping to unary functions -- we couldn't pipe, say, 
 * both the request and the response object, or the error and the data
 * argument from one function to the next without wrapping them in 
 * another object or an array. Yuck.
 *
 * So we can't return them both, but we could call another function 
 * with both! Ok, now we just wrapped them in another function instead 
 * of a plain object. :-P
 */

function use (...fns) {
  let i = 0

  return function next (...args) {
    const fn = fns[i++]
    return fn ? fn(...args, next) : args[0]
  }
}

// Contrived sample "usage":
const square = use(
  (num, next) => !num || isNaN(num)
    ? next('Please pass a valid number!') 
    : next(null, num),
  (err, num, next) => err ? (next || num)(err) : next(null, num * num),
  (err, res) => err ? console.error(err) : console.log(res)
)

square(2)

/**
 * However, we could even drive this further by allowing a function
 * to recursively call itself:
 */

function use (...fns) {
  let i = 0

  return function next (...args) {
    const fn = fns[i++]

    return (function self (...args) {
      return fn ? fn(...args, next, self) : args[0]
    })(...args)
  }
}

// Something more "useful":
const faculty = use(
  (val, next) => next(val, val),
  (res, curr, next, self) => curr === 1 
    ? next(res) 
    : self(--curr * res, curr),
  res => console.log(res)
)

faculty(4)

// MIT @ m3g4p0p