Question

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What do empty parentheses () after a function declaration do in javascript?

I am looking at some Javascript code and trying to figure out what }(); right after return num+10 means. Does that mean the function will get executed immediately? Where can I get more information about this.

function addToTen(num) {
     return function() {
          return num+10;
     }();
}
addToTen(5); // 15

Thanks,
Venn.

Answer 1

Yes, but only if it’s a function expression, which is different from a function declaration. A function declaration is how your first function is defined:

function foo(){
}

If you add () after this, you get a Syntax Error. If the function is defined as a function expression, adding a set of parenthesis immediately executes it. Functions are expressions whenever they are not defined as above, e.g.:

(function(){}());

var x = function(){}();

return function(){}();

---

Your actual example is just… odd, and pointless. It’s almost a classic closure example, something like:

function addTo(x){
    return function(y){
        return x + y;
    }
}

var addToTen = addTo(10);
addToTen(5); // 15;

but your actual example is equivalent to just:

function addToTen(num){
    return num + 10;
}

and all the extra stuff is completely unnecessary.