curry.js

/**
 * Ha! I totally fucked up. I noticed after publishing this, so instead of quietly updating this I will
 * add a bit of educational text.
 * 
 * The function below, curry_bodged, makes the mistake of saving the args in lexical scope, resulting in 
 * an ever-expanding list of arguments. Every time you call the curried function the arguments are added to 
 * the same list, and the actual applied function will probably re-use the same arguments from the first time.
 *
 * Example with the `addThree` function below:
 * addThree(1, 2, 3) // 6, args = [1, 2, 3]
 * addThree(42, 100, 48) // also 6, because args = [1, 2, 3, 42, 100, 48]
 *
 * Well. Anyways. The curry function below is correct and actually much more elegant.
 * 
 * Oh, one more thing: since this curry() function uses Function.bind(null) it will totally screw up functions 
 * using `this`.
 * But I never use `this` so I don't care for the moment. Bye now, gotta run!
 */
const curry = fn => {
  const arity = fn.length;
  return (...args) => args.length < arity 
    ? curry(fn.bind(null, ...args)) 
    : fn(...args);
};



// usage:
const addThree = curry((a, b, c) => a + b + c);
// addThree(1, 2, 3) === addThree(1)(2)(3) === addThree(1, 2)(3) === addThree(1)(2, 3) === 6

curry_bodged.js

const curry_bodged = fn => {
  let args = [];
  const arity = fn.length;
  return function inner(...xs) {
    args = args.concat(xs);
    if (args.length < arity) {
      return inner;
    }
    return fn(...args);
  };
};