Simple curry function

curry.js

// Placeholder
const _ = function _ () {}

// Curry a given function, optionally specifying
// a context on which it should finally get called
const curry = function curry (fn, thisArg) {

  // Accumulate the call arguments for the curried function
  return function accumulate () {

    // The current arguments
    const args = Array.from(arguments)

    // The accumulated call arguments
    const callArgs = Array

      // `this` is the array of call arguments so far...
      // just to avoid side effects ;-)
      .from(this)

      // Replace placeholders in the call arguments
      // with the current arguments
      .map(arg => arg === _ ? args.pop() || _ : arg)

      // Append the remaining current arguments to the
      // call arguments
      .concat(args)

    // If there are non-placeholder arguments missing
    return fn.length - callArgs.filter(arg => arg !== _).length

      // Wait for further arguments, binding the call
      // arguments so far as the accumulation context
      ? accumulate.bind(callArgs)

      // Otherwise apply the final call arguments to the 
      // function
      : fn.apply(thisArg, callArgs)

  // Start the currying with no arguments
  }.bind([])
}

Sample usage

const calc = function (a, b, c) {
  return a + b * c
}

const curried = curry(calc)

// All logs `7`
console.log(curried(1, 2, 3))
console.log(curried(1)(2)(3))
console.log(curried(1)(2, 3))
console.log(curried(1, _, 3)(2))
console.log(curried(_, 2)(1)(3))
console.log(curried(1, _)(_, 3)(2))