fn.js

// partially invoked function
function add(x, y) {
  if (arguments.length == 1)
    return function(y) {return x + y}

  return x + y
}

// pipe function for chaining function calls
function pipe() {
  var args = Array.prototype.slice.call(arguments)
  var d = args.shift()
  var f = args.shift()
  args.unshift(f(d))

  if (args.length > 1)
    return pipe.apply(this, args)

  return args[0]
}

// console.log(add(4, 2))
pipe(2, add(4), console.log)