Write a sum function that works like this: sum(a)(b) = a+b, how to understand 2 returns?

Question

JavaScript

Hello everyone, how to understand two returns? I understand that this is a return value, but in more detail? and 2 returns do closures? thanks
Write a sum function that works like this: sum(a)(b) = a+b
function sum(a ) {
return function(b) {
return a + b;
};
}
alert( sum(1)(2) ); //3
alert( sum(5)(-1) ); //4

Answer 1

This means that the sum function will return another function

function sum() {
  return function() {
  }
}