ES6 double arrow parameters (i.e. const update = x => y => { } ) [duplicate]

Question

What does double arrow parameters mean in the following code?

const update = x => y => {
   // Do something with x and y
}

How is it different compared to the following?

const update = (x, y) => {
   // Do something with x and y
}

Thanks!

Answer 1

Let's rewrite them "old style", the first one is:

const update = function (x) {
  return function(y) {
    // Do something with x and y
  };
};

While the second one is:

const update = function (x, y) {
  // Do something with x and y
};

So as you can see they are quite different, the first returns an "intermediate" function, while the second is a single function with two parameters.