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Troubleshoot custom functions

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When developing custom functions, you may encounter errors in the product while creating and testing your functions.

Important

Note that Excel custom functions are available on the following platforms.

  • Office on the web
  • Office on Windows
    • Microsoft 365 subscription
    • retail perpetual Office 2016 and later
    • volume-licensed perpetual Office 2021 and later
  • Office on Mac

Excel custom functions aren't currently supported in the following:

  • Office on iPad
  • volume-licensed perpetual versions of Office 2019 or earlier on Windows

To resolve issues, you can enable runtime logging to capture errors and refer to Excel's native error messages. Also, check for common mistakes such as leaving promises unresolved.

Debugging custom functions

To debug custom functions add-ins that use a shared runtime, see Overview of debugging Office Add-ins.

To debug custom functions add-ins that don't use a shared runtime, see Custom functions debugging.

Enable runtime logging

If you're testing your add-in in Office on Windows, you should enable runtime logging. Runtime logging delivers console.log statements to a separate log file you create to help you uncover issues. The statements cover a variety of errors, including errors pertaining to your add-in's manifest file, runtime conditions, or installation of your custom functions. For more information about runtime logging, see Debug your add-in with runtime logging.

Check for Excel error messages

Excel has a number of built-in error messages which are returned to a cell if there is calculation error. Custom functions only use the following error messages: #NULL!, #DIV/0!, #VALUE!, #REF!, #NAME?, #NUM!, #N/A, and #BUSY!.

Generally, these errors correspond to the errors you might already be familiar with in Excel. The are only a few exceptions specific to custom functions, listed here:

  • A #NAME error generally means there has been an issue registering your functions.
  • A #N/A error is also maybe a sign that that function while registered could not be run. This is typically due to a missing CustomFunctions.associate command.
  • A #VALUE error typically indicates an error in the functions' script file.
  • A #REF! error may indicate that your function name is the same as a function name in an add-in that already exists.

Clear the Office cache

Information about custom functions is cached by Office. Sometimes while developing and repeatedly reloading an add-in with custom functions your changes may not appear. You can fix this by clearing the Office cache. For more information, see Clear the Office cache.

Common problems and solutions

Can't open add-in from localhost: Use a local loopback exemption

If you see the error "We can't open this add-in from localhost," you will need to enable a local loopback exemption. For details on how to do this, see this Microsoft support article.

Runtime logging reports "TypeError: Network request failed" on Excel on Windows

If you see the error "TypeError: Network request failed" in your runtime log while making calls to your localhost server, you'll need to enable a local loopback exception. For details on how to do this, see Option #2 in this Microsoft support article.

Ensure promises return

When Excel is waiting for a custom function to complete, it displays #BUSY! in the cell. If your custom function code returns a promise, but the promise does not return a result, Excel will continue showing #BUSY!. Check your functions to make sure that any promises are properly returning a result to a cell.

Error: The dev server is already running on port 3000

Sometimes when running npm start you may see an error that the dev server is already running on port 3000 (or whichever port your add-in uses). You can stop the dev server by running npm stop or by closing the Node.js window. In some cases, it can take a few minutes for the dev server to stop running.

My functions won't load: associate functions

In cases where your JSON has not been registered and you have authored your own JSON metadata, you may see a #VALUE! error or receive a notification that your add-in cannot be loaded. This usually means you need to associate each custom function with its id property specified in the JSON metadata file. This is done by using the CustomFunctions.associate() function. Typically this function call is made after each function or at the end of the script file. If a custom function is not associated, it will not work.

The following example shows an add function, followed by the function's name add being associated with the corresponding JSON id ADD.

/**
 * Add two numbers.
 * @customfunction
 * @param {number} first First number.
 * @param {number} second Second number.
 * @returns {number} The sum of the two numbers.
 */
function add(first, second) {
  return first + second;
}

CustomFunctions.associate("ADD", add);

For more information on this process, see Associating function names with JSON metadata.

Known issues

Known issues are tracked and reported in the Excel Custom Functions GitHub repository.

Reporting feedback

If you are encountering issues that aren't documented here, let us know. There are two ways to report issues.

In Excel on Windows or on Mac

If using Excel on Windows or on Mac, you can report feedback to the Office extensibility team directly from Excel. To do this, select File > Feedback > Send a Frown. Sending a frown will provide the necessary logs to understand the issue you are hitting.

In Github

Feel free to submit an issue you encounter either through the "Content feedback" feature at the bottom of any documentation page, or by filing a new issue directly to the custom functions repository.

Next steps

Learn how to make your custom functions compatible with XLL user-defined functions.

See also