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Uncaught TypeError Cannot read properties of undefined reading easing after upgrading to 2026 Q1

Updated on Feb 17, 2026

Environment

Product	UI for ASP.NET AJAX
Version	2026.1.211

Description

After upgrading to Telerik UI for ASP.NET AJAX 2026.1.211 (2026 Q1), pages with Telerik controls throw JavaScript errors in the browser console:

Uncaught TypeError: Cannot read properties of undefined (reading 'easing')
Uncaught TypeError: $telerik.$ is not a function
Uncaught TypeError: h is not a constructor

These errors occur on pages that previously worked with an older version and use the legacy approach for disabling the embedded jQuery — a manual <script> tag in the <head> combined with explicit ScriptReference entries in RadScriptManager:

aspx

<head runat="server">
    <script
        src="https://code.jquery.com/jquery-3.7.1.min.js"
        integrity="sha256-/JqT3SQfawRcv/BIHPThkBvs0OEvtFFmqPF/lYI/Cxo="
        crossorigin="anonymous"></script>
</head>

...

<telerik:RadScriptManager runat="server" ID="RadScriptManager1" EnableEmbeddedjQuery="false">
    <Scripts>
        <asp:ScriptReference Assembly="Telerik.Web.UI" Name="Telerik.Web.UI.Common.Core.js" />
        <asp:ScriptReference Assembly="Telerik.Web.UI" Name="Telerik.Web.UI.Common.jQueryExternal.js" />
        <asp:ScriptReference Assembly="Telerik.Web.UI" Name="Telerik.Web.UI.Common.jQueryPlugins.js" />
    </Scripts>
</telerik:RadScriptManager>

Cause

Starting with the 2026 Q1 release, Telerik UI for ASP.NET AJAX initializes an isolated internal jQuery instance exposed through $telerik.$ . The controls no longer attach to the global window.jQuery object automatically.

This behavior occurs when:

EnableEmbeddedjQuery is set to false — Telerik does not load its own jQuery.
No ExternaljQueryUrl is provided — Telerik has no jQuery source to create its internal $telerik.$ instance.
The old manual ScriptReference approach is used — The legacy Core.js / jQueryExternal.js / jQueryPlugins.js script references and a standalone <script> tag in the <head> no longer wire up $telerik.$ correctly.

As a result, $telerik.$ remains undefined, the easing/animation extensions in jQueryPlugins.js cannot attach, and controls fail during initialization.

Solution

Remove the legacy <script> tag and ScriptReference entries, and use the new ExternaljQueryUrl property instead.

Option 1: Configure per page in markup

Replace the old RadScriptManager declaration:

aspx

<telerik:RadScriptManager runat="server" ID="RadScriptManager1"
    EnableEmbeddedjQuery="false"
    ExternaljQueryUrl="~/scripts/jquery-3.7.1.min.js">
</telerik:RadScriptManager>

Remove the manual <script> tag from the <head> and the <Scripts> section entirely. The ExternaljQueryUrl property handles everything automatically.

Option 2: Configure globally in web.config (recommended)

Set both properties in <appSettings> so every RadScriptManager in the application picks them up automatically — no per-page markup changes needed:

xml

<appSettings>
    <add key="Telerik.ScriptManager.EnableEmbeddedjQuery" value="false" />
    <add key="Telerik.ScriptManager.ExternaljQueryUrl" value="~/scripts/jquery-3.7.1.min.js" />
</appSettings>

Then simplify the RadScriptManager declaration to:

aspx

<telerik:RadScriptManager ID="RadScriptManager1" runat="server">
</telerik:RadScriptManager>

Remove the manual <script> tag from the <head> and the <Scripts> section.

Steps

Remove the manual jQuery <script> tag from the <head>.
Remove the <Scripts> section containing Core.js, jQueryExternal.js, and jQueryPlugins.js references from RadScriptManager.
Set the ExternaljQueryUrl property either in the RadScriptManager markup (Option 1) or in the web.config <appSettings> (Option 2).
Confirm the external jQuery script (~/scripts/jquery-3.7.1.min.js) is accessible and correctly resolves. Use app-relative paths (~/), absolute paths (/), or CDN URLs (https://code.jquery.com/jquery-3.7.1.min.js) based on your application's requirements.

The <head> tag must have runat="server" for the external jQuery script to be injected. If runat="server" is missing, the Page.Header property is null and the external jQuery script will not render.

Verification

Apply the changes above and load a page with Telerik controls.
Check the browser console for errors.
If the page works without errors, the configuration is successful.
Type in the console jQuery.fn.jquery and $telerik.$.fn.jquery to verify that the setup and the new jQuery version is loaded as expected.