Please explain these JavaScript functions

Question

These have come up in the challenges. I don't know how to google for the information because I don't know what this kind of function is called. There are parentheses around the function. Thanks in advance for explaining these to me.

//Example 1
var a = 1;
var b = 2;
(function() {
var b = 3;
a += b;
})();
alert(a + b);

?Example 2
var x = 5;
var x = (function(){
var x = 0;
return function (){
return ++x;
}();
}());
alert(x);

Answer 1

It's a function that is called as soon as it is defined. The proper term is "IIFE", or "immediately invoked function expression".

Take this piece of code:

function f() {
return 4;
}

In javascript, we can also define the same function differently:

let f = function() {
return 4;
};

Now of course we can call `f` and it returns `4`, like so:

let x = f();

But instead of going through the trouble of putting the function in a variable, we can do it in one step, like this:

let x = function(){
return 4;
}();
// x is also 4 here.

So we define a function and immediately call it. For clarity we usually add another pair of parentheses like this:

let x = (function() {
return 4;
})();

But they are not strictly necessary.

Answer 2

Perfect explanation. Thanks so much!