Question

I’m trying to understand why this alerts to true? And how I would be able to alert false without passing arguments to the callback function (if possible)?

var a = true;

function foo(callback){
    var a = false;
    callback();
}

function bar(){
    alert(a);
}
foo(bar); // Alerts true

Answer 1

This:

var a = false;

…is local to the scope of the foo function.

This:

function bar(){
    alert(a);
}

…was created in the variable scope where a = true, and as such, closed over that local variable environment, and thus over that specific a variable.

It comes down to the fact that whenever you create a function, it permanently retains the variable scope in which it was created.

It doesn’t matter if you pass that function into another environment. It will always only reference its original variable environment.

var test0 = 0;   // global variable environment

function a() {

    var test1 = 1;  // "a()" will always retain the global environment even if you
                    //      send "a()" somewhere else

    function b() {

        var test2 = 2;  // "b()" will always retain the environment of "a()" and the
                        //    global environment, even if you send "b()" somewhere else
    }
}

---

EDIT:

In order for bar to reference the variables local to foo, you could pass them in to bar as arguments:

var a = true;

function foo(callback){
    var a = false;
    callback( a );
}

function bar( a ){
    alert( a );
}
foo(bar); // now it alerts false

Example: http://jsfiddle.net/dSZ4M/

…you’ll notice that I gave the parameter in bar() the same name as the global a variable. Because parameters to the function are read before variables outside the function’s own variable environment, the a parameter “shadows” the global a variable.

As such, you can no longer read the global a from inside bar. Of course, all you’d need to do is change the name of the parameter to something else, like arg or whatever, and then you’d be able to reference both the local arg parameter and the global a variable.