JavaScript () following } [duplicate]

Question

I am studying JavaScript and I sometimes see something like this

function functionname()
{
   // Some statements
} () ;

What does () following } mean?

Thanks a lot, experts on Stack Overflow

Answer 1

That is an IIFE (immediately invoked function expression). It is used to create a new scope. For example:

var x = 10;
(function() {
   var x = 5;
}());
alert(x); // still 10

All it does is define and call the function at the same time.

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IIFEs are also used to "save" the value of a variable. For example, this code won't work:

for (var i = 0; i < buttons.length; i ++) {
    buttons[i].onclick = function() { alert(i) }
}

Every button will alert the last index when you click it, because after the loop is done i is buttons.length. To fix it, you would use an IIFE:

for (var i = 0; i < buttons.length; i ++) {
    buttons[i].onclick = (function(i) {
        return function() {
            alert(i)
        }
    })(i)
}

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More info on IIFEs