_curry.js

function curry (f) {
  return function next (a) {
    return function (b) {
      if (b) {
        return next(f(a, b))
      }
      return a
    }
  }
}

function curryLazy (f) {
  return function next (...a) {
    return function (b) {
      if (b) {
        return next(...a, b)
      }
      return a.reduce(f)
    }
  }
}

const add = (a, b) => a+b
const curryAdd = curryLazy(add)

curryAdd(1)(2)(3)(4)()

curry.js

function curry (f) {
  const next = a => b => (b === undefined ? a : next(f(a, b)))
  return next
}