Question

Can anyone please explain this code? This example is taken from javascript.info.

I don’t understand, especially the f.toString = function(){ return sum} part.

function sum(a) {

   var sum = a 

   function f(b) {
       sum += b
       return f
   }

   f.toString = function() { return sum }

   return f
}

alert( sum(1)(2) )  // 3

alert( sum(5)(-1)(2) )  // 6

alert( sum(6)(-1)(-2)(-3) )  // 0

alert( sum(0)(1)(2)(3)(4)(5) )  // 15

Answer 1

I think the author of that snippet wanted to achieve one goal, that is, something like “faking” operator overloading which is possible in other languages.

Since sum returns a function reference, we could not go like

sum(5) + 5;

That would result in something weird like "function sum() { ... }5". That is because ECMAscript calls the .toString() method on objects when invoked in a Math operation. But since he overwrites the .toString() method returning sum (which is a number), it does work again.