A curry function on JS

curry.js

const curry = (fn, arity = null) => {
  const realArity = fn.length;
  const providedArity = arity === null ? realArity :
    (arity > realArity ? realArity : arity);

  return (...args) => {
    const argsUsed = args.length > providedArity ? providedArity : args.length;
    const finalFn = fn.bind(this, ...args.slice(0, argsUsed));

    return realArity === argsUsed ? finalFn() : curry(finalFn);
  }
};

// Usage:
const testFn = (a, b, c) => [a, b, c];
curry(testFn)(4); // returns a function with arity === 2
curry(testFn)(4, 1); // returns a function with arity === 1
curry(testFn)(4)(1); // returns a function with arity === 1
curry(testFn)(4)(1)(3); // returns original function result: [4, 1, 3]