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Resolving "Function Name is not defined" Error for RadComboBox Event Handlers


Product Telerik UI for ASP.NET AJAX
Version all


When clicking on the RadComboBox, a console error stating "Function Name is not defined" (e.g., "onDropDownOpened is not defined") appears, despite attaching a JavaScript function to the OnDropDownOpened event property.


The error typically occurs when the JavaScript function is not recognized at the point when the event is triggered. Potential causes include script loading timing, incorrect script tag placement, issues with the ASP.NET page lifecycle, or typos in the function name.


To resolve the "Function Name is not defined" error, follow these troubleshooting steps:

  1. Script Loading Timing: Ensure that the <script> tag containing the onDropDownOpened function is loaded before triggering the event.
  2. Script Tag Placement: Place the <script> tag within the correct part of the page, such as within the <head> or just before the closing </body> tag, to ensure correct loading.
  3. Page Lifecycle: If using ASP.NET, verify that the script is not within an update panel or subjected to any partial page update mechanism that might prevent proper loading. You can wrap it inside RadCodeBlock tags.
  4. Typo in Function Name: Double-check the function name in both the event handler and the script to ensure there are no typos.

Here is a revised example with the script tag placement:

<!DOCTYPE html>
    <title>RadComboBox Example</title>
    <telerik:RadCodeBlock runat="server" ID="RadCodeBlock">
        <script type="text/javascript">
            function onDropDownOpened(sender, eventArgs) {
                var comboBox = sender;
                var items = comboBox.get_items();
                if (items.get_count() > 0) {
                    var firstItem = items.getItem(0);
    <form id="form1" runat="server">
        <telerik:RadComboBox ID="RadComboBox1" runat="server" OnClientDropDownOpened="onDropDownOpened">
                <telerik:RadComboBoxItem Text="Item 1" />
                <telerik:RadComboBoxItem Text="Item 2" />
                <telerik:RadComboBoxItem Text="Item 3" />

Additional Troubleshooting Steps

  • Browser Console: Check for any other errors in the browser console that might provide more context.
  • Network Tab: In the browser's network tab, verify that the script is being loaded correctly without any 404 errors.
  • Page Source: View the page source to confirm the script's presence and that it is correctly formed.

If the issue persists after following these steps, it may be necessary to review the script and event handler attachment more closely for any discrepancies or errors.

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