excel-custom-functions-sync

page_type	sample
urlFragment	excel-custom-functions-sync
products	office-excel office
languages	typescript
extensions	contentType technologies createdDate samples Add-ins 03/19/2026 4:00:00 PM
description	Shows how to create a synchronous custom function that reads a cell value in tandem with Excel's calculation process.

Create Synchronous Custom Functions in Excel (preview)

This sample shows how to create a synchronous custom function that reads a cell value in tandem with Excel calculation processes. You'll learn how to:

• Create synchronous custom functions in Excel
• Use the shared runtime

Note: Synchronous custom functions are currently in public preview and require the preview version of the Office JavaScript API. Do not use this feature in a production add-in.

How to run this sample

Prerequisites

• Download and install Visual Studio Code.
• Install the latest version of the Office Add-ins Development Kit into Visual Studio Code.
• Node.js (the latest LTS version). Visit the Node.js site to download and install the right version for your operating system. To verify if you've already installed these tools, run the commands node -v and npm -v in your terminal.
• Microsoft Office connected to a Microsoft 365 subscription. You might qualify for a Microsoft 365 E5 developer subscription through the Microsoft 365 Developer Program, see FAQ for details. Alternatively, you can sign up for a 1-month free trial or purchase a Microsoft 365 plan.

Run the add-in locally

1. Open a terminal in the Samples/excel-custom-functions-sync folder.
2. Run npm install.
3. Run npm start.

Run the add-in from the Office Add-ins Development Kit

1. Create a new project with the sample code.

   Open the Office Add-ins Development Kit extension and view samples in the Sample gallery. Select the Create button in the top-right corner of the sample page. The new project will open in a second Visual Studio Code window. Close the original VSC window.

2. Open the Office Add-ins Development Kit.

   Select the icon in the Activity Bar to open the extension.

3. Preview Your Office Add-in (F5).

   Select Preview Your Office Add-in(F5) to launch the add-in and debug the code. In the drop down menu, select the option Desktop (Edge Chromium).

   

   The extension checks that the prerequisites are met before debugging starts. The terminal will alert you to any issues with your environment. After this process, the Excel desktop application launches and opens a new workbook with the sample add-in sideloaded. The add-in automatically opens as well.

   If this is the first time that you have sideloaded an Office Add-in on your computer (or the first time in over a month), you may be prompted to delete an old certificate and/or to install a new one. Agree to both prompts. The first run requires installing dependency of this project, which might take 2~3 minutes or longer. During this time, there might be a dialog pop up at the lower right of the VSC screen. You should not interact with this dialog before the Office application launched.

4. Stop Previewing Your Office Add-in.

   Once you are finished testing and debugging the add-in, select the icon and then select Stop Previewing Your Office Add-in. This closes the web server and removes the add-in from the registry and cache.

Use the sample add-in

1. Select the Set up sample data button in the task pane to populate the active worksheet with sample values.
2. Cells B2 and B3 use the GETCELLVALUE synchronous custom function to read values from A2 and A3.
3. Change the values in A2 or A3 to see the synchronous custom function update automatically during Excel's calculation.

The add-in adds the following custom function to the workbook.

• =SyncCFSample.GETCELLVALUE("A1"): Reads the value of the specified cell, evaluated synchronously with Excel's calculation.

Explore sample files

These are the important files in the sample project.

| .eslintrc.json
| .gitignore
| .vscode/
|   | extensions.json
|   | launch.json               Launch and debug configurations
|   | settings.json
|   | tasks.json
| assets/                       Static assets, such as images
| babel.config.json
| manifest*.xml                 Manifest file
| package.json
| README.md
| RUN_WITH_EXTENSION.md
| src/                          Add-ins source code
|   | taskpane/
|   |   | taskpane.html         Task pane entry HTML
|   |   | taskpane.js           Add API calls and logic here
|   | functions/
|   |   | functions.ts          Custom function TypeScript
| tsconfig.json
| webpack.config.js             Webpack config

Details

Create the synchronous custom function

The getCellValue function in functions.ts is marked with @supportSync so that it evaluates during Excel's calculation cycle. It creates a new Excel.RequestContext, calls setInvocation with the invocation parameter, and reads a cell value. For more details see the getCellValue function in functions.ts.

Set up and use the custom function

The taskpane.js file populates sample data in column A and inserts formulas using GETCELLVALUE in column B. When you change a value in column A, the synchronous custom function re-evaluates during the Excel calculation and column B updates automatically.

Troubleshooting

If you have problems running the sample, take the following steps.

• Close any open instances of Excel.
• Close the previous web server started for the sample with the Stop Previewing Your Office Add-in Office Add-ins Development Kit extension option.
• Try to run the sample again.

If you still have problems, see troubleshoot development errors or create a GitHub issue and we'll help you.

For information on running the sample on Excel on the web, see Sideload Office Add-ins to Office on the web.

For information on debugging on older versions of Office, see Debug add-ins using developer tools in Microsoft Edge Legacy.

Make code changes

Once you understand the sample, make it your own! All the information about Office Add-ins is found in our official documentation. You can also explore more samples in the Office Add-ins Development Kit. Select View Samples to see more samples of real-world scenarios.

If you edit the manifest as part of your changes, use the Validate Manifest File option in the Office Add-ins Development Kit. This shows you any errors in the manifest syntax.

Engage with the team

Did you experience any problems with the sample? Create an issue and we'll help you out.

Want to learn more about new features and best practices for the Office platform? Join the Microsoft Office Add-ins community call.

Copyright

Copyright (c) 2026 Microsoft Corporation. All rights reserved. This project has adopted the Microsoft Open Source Code of Conduct. For more information, see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.