The first parameter "callback" in this case is really unused. The return statement contains a usage of "callback" parameter that is a parameter of the function wrapping the usage.

I'm not sure, I simplified it to this:


var a = (function(callback) {
    return false ||
            function(callback) {
                alert(callback);
            };
})(); 

a(2);


And alert window contains 2. I think that because on the last line the function is called immediately, the first parameter is passed to the second one.

Thanks, I get it:). Unfortunately I don't have doubt how to solve this. So lets keep it as reminder.

The correct code would be:

```
window.requestAnimFrame = (function() {
    return window.requestAnimationFrame ||
            window.webkitRequestAnimationFrame ||
            function(callback) {
                window.setTimeout(callback, 1000 / 60);
            };
})();
```


Note that this will also give "2":

```
var a = (function() {
    return false ||
            function(callback) {
                alert(callback);
            };
})(); 

a(2);
```

This would also work the same:
```
var a = function(callback) {
               alert(callback);
        };
a(2);
```

Note that you simply define an anonymous function and "save" it to a variable `a` (in first case you return the function as a value). And callback in original code is really unused (as it is shadowed by function argument).

This will give you "outside":
```
var a = (function(callback) {
    callback = "inside"
    return false ||
            function(callback) {
                console.log(callback);
            };
})(); 

a("outside");
```

This will give you `undefined` (because you haven't passed any parameters) and "outside":
```
var a = (function(callback) {
    return false ||
            function(callbackInside) {
                console.log(callback);
                console.log(callbackInside);
            };
})(); 

a("outside");
```

This will give you "calling anon" and "outside":
```
var a = (function(callback) {
    return false ||
            function(callbackInside) {
                console.log(callback);
                console.log(callbackInside);
            };
})("calling anon"); 

a("outside");
```

You're right of course, the "proof" in comment #2 does not really prove anything :). Thank you for noticing that.