[Javascript] Understand Curry

Let's see an example:

const add = a => b=> a + b;
const inc = add(1);

const res = inc(3) // 4

This is a very basic currying example. 

We wrote a function 'add' take a param 'a' return a function which take param 'b', finally return a + b;

We can based on 'add' function to create another function 'inc'. This approach is good for resuability. But there is one problem, because you cannot do:

add(1,3)

To solve the problem we need to write a 'curry' function, the basic idea is:

1. Check the passed in function, how many params it should takes, in our case 'add' function take 2 params.

2. 'curry' should return another function, inside function it should check, if length of params is the same as function's params, in our case is:

add(1,3)

Then we invoke function immediately. (Using .call for immediately invoke) 

3. Otherwise, we should still return a function with param partially applyied. (using .bind for partially apply)

function curry(fn) {
  // The number of fn's params
  // fn = (a, b) => a + b
  // then length is 2
  const arity = fn.length;
  console.log(arity)
  return function $curry(...args) {
    console.log(args, args.length)
    if (args.length < arity) {
      // partially apply
      // in case of add(1)(2)
      return $curry.bind(null, ...args);
    }
    // immediately invoke
    // in case of add(1,2)
    return fn.call(null, ...args);
  };
};

const add = curry((a, b) => a + b);
const inc = add(1);
const res = inc(2) // 3