Javascript global variable scope issue

Question

I am running into a strange scope issue with Javascript (see JSFiddle):

var someGlobal = 3;

function someF() {
    // undefined issue
    alert(someGlobal);
    var someGlobal = 5;
    // Displays 5
    alert(someGlobal);
}

function someF2() {
    // Displays 3, why?
    alert(someGlobal);
}

someF();
someF2();

Why doesn't Javascript throws an undefined issue in someF2()? How come someF2() can access the someGlobal, and someF() not? How can I make sure a global variable is accessible in a function?

Remark:

In both cases, the functions start by calling alert(someglobal), why does one function throw an undefined issue and the other not?

Quentin · Accepted Answer · 2013-01-02 12:38:43Z

7

someF creates a new (locally scoped) variable called someGlobal (which masks the global someGlobal) and assigns a value to it. It doesn't touch the global someGlobal (although cannot access it because there is another variable with the same name in scope).

var statements are hoisted, so someGlobal is masked for all of someF (not just after the var statement). The value of the local someGlobal is undefined until a value is assigned to it.

someF2 access the original (untouched) global someGlobal.

edited Jan 2, 2013 at 12:38

answered Jan 2, 2013 at 12:33

Quentin

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4 Comments

Talha Akbar Over a year ago

+1 for best answer but describe more that global variable gets overwrite by local variable in function :D

Quentin Over a year ago

It doesn't get overwritten, it gets masked.

Talha Akbar Over a year ago

well can you give me a little explanation about both! i haven't done research on it! For only that function it gets overwritten or not?

Jérôme Verstrynge Over a year ago

Are you saying that because someF() declares a local variable called someGlobal after the first alert, this impacts this first alert() by having it throw the undefined issue?

Ali · Accepted Answer · 2013-01-02 12:33:09Z

4

Since you are declaring a local variable with the same name. So it assigns the value to the local variable. Just remove the var from var someGlobal in someF() and it should be fine.

var someGlobal = 3;

function someF() {
    // undefined issue
    alert(someGlobal);
    someGlobal = 5; // <-- orignially var someGlobal = 5
    // Displays 5
    alert(someGlobal);
}

function someF2() {
    // Should display 5 now
    alert(someGlobal);
}

someF();
someF2();

answered Jan 2, 2013 at 12:33

Ali

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Comments

Paul Collingwood · Accepted Answer · 2013-01-02 12:33:22Z

1

someF2 displays 3 because it still is 3.

In someF() you create a new variable that happens to have the same name as someGlobal. That does not do anything to the original someGlobal, it just creates a new variable locally to function someF that goes away when that function ends.

So you have local variables (e.g. created inside someF with var) and global ones.

answered Jan 2, 2013 at 12:33

Paul Collingwood

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2 Comments

Jérôme Verstrynge Over a year ago

Ok, but why do I get an undefined issue in one case and not the other? That's my question.

Paul Collingwood Over a year ago

I think Quentin has now explained that.

Dustin Kingen · Accepted Answer · 2013-01-02 13:01:21Z

-1

Here is an example of how to use both the local and global variable inside someF by using this.

var someGlobal = 3;

function someF() {

    // Displays 3
    alert(someGlobal);
    this.someGlobal = 5;
    someGlobal = 5;
    // Displays 5
    alert(this.someGlobal);
}

function someF2() {
    // Displays 5
    alert(someGlobal);
}

someF();
someF2();

answered Jan 2, 2013 at 13:01

Dustin Kingen

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