Why undefined var is not added to window object JavaScript?

Question

As far as I know the following declaration will not add any value to the variable aa:

var aa = undefined;

function a () {
    var aa;
    console.log(aa);  // here aa is still undefined
    if(!aa) {
        aa = 11;  // should add to the globle scope (window Object)
        bb = 12;  // should add to the globle scope (window Object)
    }
    console.log(aa);
    console.log(aa);  // should be 11
    console.log(bb);  // should be 12
}

Now if I want to use access the vars aa and bb, I can get access only bb not aa. My question is why aa cannot be accessed from outside, because in the declaration I haven't assigned any value to it and it is still undefined?

Thank you.

you have redefined aa to be in the function scope, and since you didn't assign a value, it has assigned undefined — Jonathan de M.
– Jonathan de M., Commented Jun 14, 2013 at 6:32
@JonathandeM.: So do you mean undefined is the value that is being assigned to the variable? — me_digvijay
– me_digvijay, Commented Jun 14, 2013 at 6:42

Jonathan de M. · Accepted Answer · 2013-06-14 07:19:39Z

1

Look at my comments

var aa = undefined; // global scope

function a () {
    if(true) { // useless
        var aa; // declare aa in the function scope and assign undefined
        // to work on the global aa you would remove the above line
        console.log(aa);  // here aa is still undefined
        if(!aa) {
            aa = 11;  // reassign the local aa to 11
            bb = 12;  // assign 12 to the global var bb
        }
        console.log(aa); // aa is 11
    }
    console.log(aa);  // still in the function scope so it output 11
    console.log(bb);  // should be 12
}
console.log(aa) // undefined nothing has change for the global aa

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edited Jun 14, 2013 at 7:19

answered Jun 14, 2013 at 6:36

Jonathan de M.

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4 Comments

me_digvijay Over a year ago

So do you mean undefined is the value that is being assigned to the variable in my case?

Jonathan de M. Over a year ago

yes the value by default is undefined unless you assign manually a value, var a = undefined is equal to var b; a === b

me_digvijay Over a year ago

Thank you for clearing my misconception. I always thought of var a as nothing is there. But now what I can understand is there may occur a variable 'a' which is inside this scope but no value has been assigned to it. Please correct me if I am wrong.

Jonathan de M. Over a year ago

undefined is a special value which means nothing has been defined. So not defining or defining undefined is kinda the same. About the scope here a good ebook

uber5001 · Accepted Answer · 2013-06-14 06:33:54Z

0

Try removing var aa; from within your function.

What's happening here is function scope. You've declared aa as a local variable within function a. the local variable IS getting set to 11.

answered Jun 14, 2013 at 6:33

uber5001

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