JS currying example

curry.js

// the prime hero here
function curry(func, expectedCount, ...args) {
  function _check(arr) {
    return arr.length >= expectedCount ? _result(arr) : _curry.bind(null, arr)
  }

  function _curry(prev, ...rest) {
    return _check(prev.concat(rest))
  }

  function _result(arr) {
    return func.apply(this, arr)
  }

  return _check(args)
}

// 2 simple "workers" to apply currying on
function multiplyAll(...arr) {
  return arr.reduce((p, c) => p * c, 1)
}

function sayTo(type, target, end = '!') {
    return `${type}, ${target}${end}`
}

// helper logger
function log(...args) {
    console.log(args.join('\n'))
}

// definitions
const multiply = curry(multiplyAll, 2)                   // uses `multiplyAll` worker, expects at least 2 arguments
const double = multiply(2)                               // multiples by 2
const triple = multiply(3)                               // multiples by 3
const say = curry(sayTo, 2)                              // uses `sayTo` worker, expects at least 2 arguments
const sayHello = say('Hello')                            // says "Hello, [argument]!"
const sayHelloBabe = () => sayHello('babe')              // says "Hello, babe!"
const sayGoodBye = say('Good bye')                       // says "Good bye, [argument]"
const sayVeryGoodBye = curry(sayTo, 2)('Very good bye')  // says "Very good bye, [argument]"

// running test
log(
    multiply(1.5)(1.5),
    double(2),
    multiply(2, 3),
    triple(2.33),
    multiply(3)()(3),
    curry(multiplyAll, 3, 2, 3, 2),
    double(2, 4),
    multiply()(4)()(5),
    double(triple(4)),
    triple(3, 3),
    multiply()(3)(4, 5),
    curry(multiplyAll, 3)(10)(20)(30),
    sayHello('world'),
    sayHello()('cousin'),
    sayHelloBabe(),
    sayGoodBye('cruel world'),
    sayVeryGoodBye('for ever', '!!!'),
    '-= END =-'
)

output

2.25
4
6
6.99
9
12
16
20
24
27
60
6000
Hello, world!
Hello, cousin!
Hello, babe!
Good bye, cruel world!
Very good bye, for ever!!!
-= END =-