Sum with closures

Write function sum that works like this: sum(a)(b) = a+b.

Yes, exactly this way, using double parentheses (not a mistype).

For instance:

          sum(1)(2) = 3
sum(5)(-1) = 4
        

For the second parentheses to work, the first ones must return a function.

Like this:

function sum(a) {

  return function(b) {
    return a + b; // takes "a" from the outer lexical environment
  };

}

alert( sum(1)(2) ); // 3
alert( sum(5)(-1) ); // 4