Why does ASP.NET sometimes set my JavaScript object to null?

There is a <script> in my code that includes the following line:

var tbl = document.getElementById("<%= this.tblSelection.ClientID %>");

Despite this, when the script runs, tbl always ends up being set to null.

The table has been defined like this:

<asp:Table ID="tblSelection" runat="server" CellPadding="2" CellSpacing="0"
    cols="1" style="position: absolute; top: 0%; right: 0%">

Both the script and the table are located within the same master page file.

What could be the issue causing this problem?

EDIT: I forgot to mention that this script is executed on onload.

asp.netjavascriptnullgetelementbyid

Answer №1

It seems like your JavaScript code is running before the table on your webpage has finished rendering. As a result, when you try to use the getElementByID function, it cannot locate the table because it hasn't been created yet!

You can test this by following these steps:

<head runat="server">
<title></title>

<script language="javascript">

    function showTable() {

        var tbl = document.getElementById("<%= this.tblSelection.ClientID %>");

        alert(tbl);
    }

    showTable();

</script>

Upon loading the page for the first time, you will see "null" displayed. However, if you add a button that triggers the method again, you will notice that tbl now contains the table.

<input type="button" value="Check Table" onclick="showTable();" />

Answer №2

Regarding the Javascript timing, both MrGumbe and Valera are correct. However, I have encountered this issue for a different reason. In some cases, server-side logic has set tblSelection.Visible=false, causing it to not be rendered in the browser at all. As a result, client-side code similar to yours searches for the ID and encounters an issue.

Answer №3

Alice is right. The chart has not been processed/created yet. Consider transferring the getElementById code to the document.ready event. By the way, jQuery offers a convenient wrapper for document events and more.

Check out this code snippet:

$(document).ready(function() {
    $get('chart-id').performAction();
});

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