A JavaScript IIFE prevents leaks to the global space as a closure? Is that the correct concept?

Question

I see in the code or sometimes people talk about it, for some JavaScript code:

(function() {
    var something;

    function someFunction() { 
        // some code here
    }

    // do something

}());

That's an "Immediately Invoked Function Expression", or IIFE. I often hear people say, "yeah, do it in a closure", or "do this in a closure" in the code comment -- as if a closure is "protecting the leak to the global space."

But is that the correct concept? I think it is a local scope, or an anonymous local scope, that is shield any local variables from leaking to the global scope. It really has nothing to do with a closure, which is a function with a scope chain. Sure, the anonymous function used for the IIFE is a closure, but it is not relevant here. If you say you want a closure, it is because you want the access to the current scope (and all scopes in scope chain). To say, shielding local variable to the global scope "by using a closure", is not a correct concept, is it?

Update: In any language that doesn't have closure, such as C, you can still do the exact same thing of shielding any local variables to leak into the global space. So it is not "closure" that is doing the job.

Answer 1

I think it is a local scope, or an anonymous local scope, that is shield any local variables from leaking to the global scope.

Yes, that's precisely what it is. And because JavaScript doesn't actually have block scopes, the only way you can implement it is with functions. Hence, IIFEs.

To say that you are using a closure here is true, because the function expression creates a closure. Without that, your "local scope" would have no access to any other variables in the scope in which it is found. However, this is again something of an implementation detail — it's not core to the problem at the design level.