How to populate html element using javascript prior to page_load()?

Question

Maybe a chicken/egg problem.

Based on this answer: https://stackoverflow.com/a/27190520/1438215

I want to use javascript to populate the value of an asp.net hidden field (as soon as it's possible), and then access the value of that populated field in the server-side Page_Load event.

Sample:

aspx portion:

<div id="div_BrowserWindowName" style="visibility:hidden;">
    <input type="hidden" name="ctl00$ctl00$BodyContent$MainContent$hf_BrowserWindowName" id="BodyContent_MainContent_hf_BrowserWindowName" />
</div>
<script type="text/javascript">
            function PopBrowserWindowName() {
    if (typeof window.name != undefined) {
        if (window.name == '') {
            var d = new Date();
            window.name = '_myWnd_' + d.getUTCHours() + d.getUTCMinutes() + d.getUTCSeconds() + d.getUTCMilliseconds();
        }
        var eDiv = document.getElementById('div_BrowserWindowName');
        var e = eDiv.getElementsByTagName('input')[0];
        e.value = window.name;
        alert(e.value);
    }
          }

    window.onload = PopBrowserWindowName();
</script>    

aspx.cs portion (page_load) event:

if (hf_BrowserWindowName != null)
       {string winID = hf_BrowserWindowName.Value;}

This does works during postbacks, but does not work on the page's initial load.

Answer 1

Prior to Page_Load , there is no HTML or JavaScript because the Response has not been sent to the client yet. You need to learn the ASP.NET Page Lifecycle .

You can use server side code to populate the information, or after the page is sent to the client you can have some JavaScript populate the information.