Question

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What do parentheses surrounding a JavaScript object/function/class declaration mean?

I have found the following code in a website .

var testModule = (function(){

    var counter = 0;

    return {

       incrementCounter: function() {

            return counter++;

        },

        resetCounter: function() {

            console.log('counter value prior to reset:' + counter);

            counter = 0;

        }

    };

})();

So it follows the syntax var a = (blah balh..)()

What does it actually mean? What is the meaning of variable declaration like a =()()..

Answer 1

It’s defining a single-use function and executing it immediately. The code you provided is named the Module Pattern — see here for more information about its properties: http://www.yuiblog.com/blog/2007/06/12/module-pattern/

A normal function might be created like this:

var f1 = function() {
  console.log('bar');
};

And you could subsequently call it like so:

f1();

But in the example you provided, the function is both defined and executed once, and that function returns an object with two functions: incrementCounter and resetCounter. You can call them like so: testModule.incrementCounter() and testModule.resetCounter()

The Module Pattern is useful when you have a single object and you want to encapsulate some properties which are only available to the functions defined in the closure.