Closure in Javascript with multiple brackets

Question

Could any one explain how this function alert, when more no of brackets of parameters are passed. I could not able to understand it clearly.

function sum(a) {

          


        

Answer 1

The sum and f functions always return the f function, you can invoke it infinite times without getting a result other than the function.

The value is only returned by the overwritten toString method of f (which actually returns a number):

console.log( sum(1)(2) ) // Function(){}
console.log( sum(1)(2).toString() ) // 3

The alert function implicitly invokes the toString method when it casts its arguments to strings.