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JavaScript Errors

This page provides solutions for common JavaScript errors you may encounter while working with Telerik UI for Blazor components.

TelerikBlazor was undefined

You may get runtime error messages in the browser console similar to the following:

  • Could not find 'TelerikBlazor.getLocationHost' ('TelerikBlazor' was undefined).
  • Microsoft.JSInterop.JSException: Could not find 'TelerikBlazor' in 'window'.

If you get such errors, the reason may be:

Missing File

You can check if this is the case by inspecting the Network tab of your browser console to see if it is returning successfully. There are a few common causes for the JS Interop file to be missing:

  • The application is missing references to the needed assets.

  • Network setup blocks access to the cloud, and thus, to our CDN. If this is the case, you have two options:

    • Discuss the case with your network administrators to have our CDN allowed.
    • Use static assets from the app folder to avoid going to the cloud.
  • The application was upgraded from a trial to a commercial license, but the path to the file was not. See the Upgrade from Trial to Commercial section for details.

  • Static assets are not enabled on the server project, or the hosting environment does not work well with them. You need to ensure that the static assets are available after the build and on the deployment server - they are in the local NuGet cache, under a path similar to C:\Users\<theUser>\.nuget\packages\telerik.ui.for.blazor\<theVersion>\staticwebassets. It is also possible that the hosting server does not support static assets or has issues with them and you may need to confirm this with the hosting provider or by testing with a simple package having a static asset to see whether it gets returned.

    • Clearing the NuGet cache, then running a Clean and Rebuild on the solution may fix the problem in case something went wrong with fetching the NuGet package. In case Clean does not clean up the bin and obj folders, you can also delete them manually before Rebuild.
  • A problem occurs during the deployment. See the Deployment Troubleshooting article for more details.

Defer Attribute

Sometimes, the JS Interop file is referenced correctly and returns successfully, but occasionally you get the error. This indicates a timing issue (for example, low machine performance or slow network) that causes the script to load and be parsed too late, after it is needed.

The solution is to:

  1. Remove the defer attribute of the <script> tag.
  2. Move the <script> tag after the Blazor framework script (_framework/blazor.server.js or _framework/blazor.webassembly.js).

Generally, defer improves the performance of your app by not making the script a blocking resource.

Remove defer and move the Telerik Blazor script tag

    <!--<script src="_content/Telerik.UI.for.Blazor/js/telerik-blazor.js" defer></script>-->
    <script src="_framework/blazor.....js"></script>
    <script src="_content/Telerik.UI.for.Blazor/js/telerik-blazor.js"></script>

Blazor Autostart

If removing the defer attribute above does not help, you can disablе the automatic client-side initialization of Blazor.


By default, TypeScript results in compiled code that needs the exports object, and that is not available in Blazor by default, so it throws an error. A common workaround for that (defining an empty exports object) causes errors from the Telerik JS Interop files. You can read more about the errors and the solutions in the TypeScript Exports error breaks Telerik Blazor Knowledge Base article.

init[Component] was undefined

The error message may mention a component or feature initialization method, for example:

  • Error: Microsoft.JSInterop.JSException: Could not find 'initGrid' in 'window.TelerikBlazor'.
  • Error: Could not find 'TelerikBlazorPopup' in 'window'.
  • Error: Could not find 'TelerikBlazor.columnResizableSetColumns' ('columnResizableSetColumns' was undefined).
  • Any error referring to a Telerik component or feature that cannot be found in the JS code.

Such an error means that the telerik-blazor.js script file version does not match the NuGet package version. As a result, the script does not include all components, features or correct method names.

If you use our CDN to load the script file, make sure the file URL matches the package version. If you load the script as a local file from the wwwroot folder, then replace the file. See the Upgrade Process article for details.

Another common reason is browser caching, if the file comes from the static NuGet assets or a local folder. Clear the browser cache or "hard refresh" the page to fix that. A real server should look at the modified date of those files and serve them fully when they have been upgraded, so in a production environment this should not occur, but a development server is more likely to return a "not modified" response even after the file was updated.

Cannot read properties of null (reading 'addEventListener')

The error message may also mention removeEventListener instead of addEventListener.

There are two known reasons for this JavaScript error.

One is related to UI for Blazor update in WebAssembly (WASM) projects. See the article TypeError: Cannot read properties of null (reading 'addEventListener').

Another possible cause is a race condition during fast multiple recreations of components, or fast navigation. If this happens, our JavaScript code may try to access a DOM element that no longer exists. The solution is to avoid fast duplicate UI refresh, for example:

  • Do not call StateHasChanged() inside EventCallback methods (e.g. Button click handlers). Blazor executes StateHasChanged() automatically in such cases.
  • Throttle the user behavior, so that rapid subsequent navigation is not possible.

KeyNotFoundException: The given key inputElementValue was not present

The full exception message is System.Collections.Generic.KeyNotFoundException: The given key 'inputElementValue' was not present in the dictionary.

This error indicates that the app is using an old or wrong version of the telerik-blazor.js file, for example, after a component version upgrade.

Object doesn't support property or method 'assign'

Under IE, you may get errors similar to Object doesn't support property or method 'assign' or errors that relate other modern JS features that are not supported under IE. The reason is that we use modern code that may not work under IE - it is not one of the browsers we support, and WebAssembly does not work on it anyway, so modern Blazor apps won't run on IE regardless.

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