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Asked: 2026-05-14T06:04:02+00:00

(creating a separate question after comments on this: Javascript redeclared global variable overrides old

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(creating a separate question after comments on this: Javascript redeclared global variable overrides old value)

I am creating a globally scoped variable using the square bracket notation and assigning it a value inside an external js file.

In another js file I declare a var with the same name as the one I just created above. Note I am not assigning a value. Since this is a redeclaration of the same variable the old value should not be overriden as described here: http://www.w3schools.com/js/js_variables.asp

Create 2 javascript files with the following content :
Script1

//create global variable with square bracket notation
window['y'] = 'old';

Script2

//redeclaration of the same variable
var y;

if (!y) y = 'new';

alert(y); //shows New instead of Old in IE

Include these 2 files in your html file

<html>
 <head></head>
 <body>

  <script type="text/javascript" src="my.js"></script>
  <script type="text/javascript" src="my2.js"></script>

 </body>
</html>

Opening this page in Firefox and Chrome alerts ‘old’ which is the expected behavior. However in IE 8 the page will actually alert ‘new’

Any ideas on why this happens on IE ?

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1 Answer

  1. Editorial Team

    If you expect y to be global, you can just drop the var y line altogether in your second file.

    The reasoning behind this is that since you want y to be global anyway, just treat it like its a global and already declared. JavaScript’s side-effect of making variables global when declared without the var prefix plays to your favor in this situation. Tested in IE8, this works just fine.

    Edit: As to why this happens, I’d chalk it up to it just being a bug in a combination of IE’s handling of globals across files and declaration hoisting. Really, though, you should only be declaring any variable, but especially a global, in one place. Your problem can be avoided by following this rule of thumb.

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