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Asked: 2026-06-16T18:11:28+00:00

I noticed recently in my ASP.NET web application that if I set my User

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I noticed recently in my ASP.NET web application that if I set my User Agent to an empty string (using a FireFox plug-in to spoof the user agent), then ASP.NET will not generate the javascript required to perform postbacks. More specifically, if you try calling the __doPostBack(a, b) function from your javascript, you will get an error saying that function is undefined.

I understand that every browser has a user agent, so this won’t come up that often, but the essence of the problem still exists: there are cases in which an unrecognized or malformed user agent can render your web application unusable if you rely on postbacks.

This is similar to this question: ASP.net not generating javascript for some User Agents, but if I’m reading it right it looks like you’d fix each unrecognized user agent case by case and mask it as another browser. My concern is less with an individual user agent, and more so with the overall fact that certain user agents won’t be able to use my application and I won’t know it because the error happens in javascript and not on the server.

Does anyone know of a way that I can force ASP.NET to always generate the required javascript?

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1 Answer

  1. Editorial Team

    If you set empty to the User Agent, or if you use an unknown User Agent string, then asp.net reads the Default.browser file from Browsers directories and there you can define how to you like to act on this cases.

    The lines that you need to change and on this cases use javascript is that ones:

            <capability name="jscriptversion"       value="5.6" />
        <capability name="javascript"           value="true" />
        <capability name="javascriptversion"    value="1.5" />

    on the default, the javascript line if false. I like to comment here that I am not so sure if you really need to change it, if some one spoof the user agent, let it do it. If he likes your site and need to use it, then is better let you know how to handle the browser that he use. From the other hand you must take care if this is possible, your site to work even with out javascript, at least for most of the actions you do.

    Is better to avoid link buttons (that use the _doPostBack) and use post back buttons for example. I know that GridView and other use also the _doPostBack for paging… ok with that, but is better one good site to been able to work even with out javascript.

    Some similar question:
    __doPostBack is undefined on DotNetNuke website for IE 10

    and this blog: Bug and Fix: ASP.NET fails to detect IE10 causing _doPostBack is undefined JavaScript error or maintain FF5 scrollbar position

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