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Editorial Team
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Asked: 2026-06-10T17:33:30+00:00

When defining variable directly, it works. Like the following code, the background color of

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When defining variable directly, it works. Like the following code, the background color of body will be light green in IE and will be light blue in non-IE browsers.

<html>
<body>
    <script>
        if (window.attachEvent) {
            var YourBrowserIsIE = true;
        }

        if (YourBrowserIsIE) {
            document.body.style.backgroundColor = 'lightgreen';
        }
        else {
            document.body.style.backgroundColor = 'lightblue';
        }
    </script>
</body>
</html>

However, sometimes the variable need to be defined using eval(), like the following, but the result will show an error saying that YourBrowserIsIE is undefined in non-IE browsers.

if (window.attachEvent) {
    eval('var YourBrowserIsIE = true;');
}

Yes, I know I can predefine var YourBrowserIsIE = false; for non-IE browsers or change the if statement to if (typeof YourBrowserIsIE != 'undefined'), but I want to keep the code as minimum as possible.

So is there a solution to use eval() to define the variable and check the variable using the straightforward if (YourBrowserIsIE) without showing any error in non-IE browsers?

== EDIT ==

Sorry for being unclear. The situation of using eval() being mentioned above is actually for detecting IE version. Please see the following code.

<html>
<body>
    <script>
        if (window.attachEvent) {
            var version = /msie (\d+)/i.exec(navigator.userAgent)[1];
            eval('var YourBrowserIsIE' + version + ' = true;');
        }

        if (YourBrowserIsIE9) {
            document.body.style.backgroundColor = 'lightgreen';
        }
        else {
            document.body.style.backgroundColor = 'lightblue';
        }
    </script>
</body>
</html>
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1 Answer

  1. Editorial Team

    but I want to keep the code as minimum as possible

    Wouldn’t that be window.YourBrowserIsIE = window.attachEvent; then?

    I see 2 advantages it it:

    1. It’s minimal
    2. It doesn’t need eval

    Seeing your code, i’d suggest to not use YourBrowserIsIE at all, but use:

    document.body.style.backgroundColor = window.attachEvent 
                                       ? 'lightgreen' : 'lightblue';

    And seeing your edits, that could/would be:

    document.body.style.backgroundColor = 
              +((/msie (\d+)/i.exec(navigator.userAgent)||[0])[1]) === 9 
                ? 'lightgreen' : 'lightblue';

    And if it has to be a reusable variable, I come back to solution 1 whith a twitch:

    window['YourBrowserIsIE'+((/msie (\d+)/i.exec(navigator.userAgent)||[0])[1]] 
       = true;
document.body.style.backgroundColor = window.YourBrowserIsIE9 ?
                                       ? 'lightgreen' : 'lightblue';
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