Why is this javascript function inside parenthesis?

Question

I was looking for a solution to a problem and I found this aswer https://stackoverflow.com/a/6962808/2724978

The function written is:

(function loop() {
    var rand = Math.round(Math.random() * (3000 - 500)) + 500;
    setTimeout(function() {
            doSomething();
            loop();  
    }, rand);
}());

My question is, why is the function insite parenthesis an also having () at the end? It got me a bit confused, never seen this before.

The 2 parenthesis at the end is to call the loop function. The block is between in parenthesis is a special notation to make everything inside private (not accesible from outside). — Walfrat, Mar 04 '16 at 12:05
If you type `function loop() { ... };` then that **defines** the function. It is not *executed* yet. Invoking the function is done by `()` (which is argument list) and that actually executes the function. You place a name before `()` and you get `loop()` which tells the JS engine to execute the function. — Mjh, Mar 04 '16 at 12:06
@Walfrat: the parens do nothing to make what's inside any more or less private. — RemcoGerlich, Mar 04 '16 at 12:08
It is called immediately-invoked function expression. The code will executed immediately but has its own private scope. — d-bro82, Mar 04 '16 at 12:08
Possible duplicate of [JavaScript IIFE](http://stackoverflow.com/questions/21781551/javascript-iife) — Evan Trimboli, Mar 04 '16 at 12:08
@RemcoGerlich Yes i know i was more telling about the whole block with the parenthesis and the function call. — Walfrat, Mar 04 '16 at 12:10

score 3 · Answer 1 · answered Mar 04 '16 at 12:05

This is an example of an Immediately Invoked Function Expression.

The link above will explain all you need to know, but essentially there are two things happening.

Wrapping within parenthesis tells the parser to treat everything within them as an expression.
Calling the expression (with ()) immediately afterwards ensures the scope of that expression is sealed.

score 1 · Answer 2 · answered Mar 04 '16 at 12:10

The snippet creates an anonymous function, and immediately calls it.

The problem with doing without parens,

// This is a syntax error
function () {
  something;
}();

is that that is a syntax error; because the statement starts with 'function', the parser expects a function name to follow. Wrapping it all in parens makes it syntatically legal to define an anonymous function there.

// This is valid syntax
(function () {
  something;
}());

Why is this javascript function inside parenthesis?

2 Answers2