In JSFiddle, why is this global variable not defined on `window`?

Question

var a = 1;  

function b() {  
    var a = 10;  
    alert(window.a);  
}  

b();

Why is a is undefined here? It's already defined in global namespace, i.e., window. (See the fiddle for an example of this unexpected behavior.)

I mean if i write 1st statement without var then a = 1 but with var a = undefined. — Sushant Prasad, Oct 12 '15 at 11:46
Is this whole code running inside another scope? Perhaps a jsfiddle with its default (wrap in onload) settings? For example: [default settings](http://jsfiddle.net/syvbnqdw/), and [changed to nowrap, place in head](http://jsfiddle.net/syvbnqdw/1/) (this one works as expected). — James Thorpe, Oct 12 '15 at 11:50
Its works for me on chrome and ie11 with an alert of `1`. Did you use another browser? Was the code encapsulate in other bracket? — SerCrAsH, Oct 12 '15 at 11:55
If you remove the window and just call it alert(a); it works on all the browsers. There is a problem that you assign the global variable inside inner scope and try to call the global variable in a scope that has 2 - "a" parameters ;] — dlght, Oct 12 '15 at 12:01

score 4 · Accepted Answer · edited May 23 '17 at 12:08

If you're running this code in a fiddle that does not have the location set to "No wrap", or any circumstance in which you're not at the top-level scope, your outer a is not the global variable window.a. Consider a simple example where your code is wrapped inside a function called wrappingFunc:

// THIS would be the global `a`, outside `wrappingFunc`
var a = "now the global a is defined";

function wrappingFunc() {
    // this is NOT the global `a`
    var a = 1;  

    function b() {  
        var a = 10;  
        alert(window.a);  
    }  

    b();  
}

wrappingFunc();

This is exactly what JSFiddle does when you set the location to onLoad or onDomready. (See What is the difference between onLoad, onDomready, No wrap - in <head>, and No wrap - in <body>?) My wrappingFunc example is in actuality an onload or ondomready listener function, which prevents the keep from running in a global context.

Yes, I was running this code on jsfiddle. I was not aware that jsfiddle is wrapping the code. — Sushant Prasad, Oct 12 '15 at 14:25

In JSFiddle, why is this global variable not defined on `window`?

1 Answers1

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