Create custom functions in Excel (Preview)

Custom functions (similar to user-defined functions, or UDFs), enable developers to add any JavaScript function to Excel using an add-in. Users can then access custom functions like any other native function in Excel (such as =SUM()). This article explains how to create custom functions in Excel.

The following illustration shows you how an end user would insert a custom function into a cell. The function that adds 42 to a pair of numbers.

custom functions

Here’s the code for the same custom function.

function ADD42(a, b) {
    return a + b + 42;

Custom functions are now available in Developer Preview on Windows, Mac, and Excel Online. Follow these steps to try them:

  1. Install Office (build 9325 on Windows or 13.329 on Mac) and join the Office Insider program. (Note that it isn't enough just to get the latest build; the feature will be disabled on any build until you join the Insider program)
  2. Clone the Excel-Custom-Functions repo and follow the instructions in the to start the add-in in Excel, make changes in the code, and debug.
  3. Type =CONTOSO.ADD42(1,2) into any cell, and press Enter to run the custom function.

See the Known Issues section at the end of this article, which includes current limitations of custom functions and will be updated over time.

Learn the basics

In the cloned sample repo, you’ll see the following files:

  • customfunctions.js, which contains the custom function code (see the simple code example above for the ADD42 function).
  • customfunctions.json, which contains the registration JSON that tells Excel about your custom function. Registration makes your custom functions appear in the list of available functions displayed when a user types in a cell.
  • customfunctions.html, which provides a <Script> reference to the JS file. This file does not display UI in Excel.
  • customfunctions.xml, which tells Excel the location of the HTML, JavaScript, and JSON files; and also specifies a namespace for all the custom functions that are installed with the add-in.

JSON file (customfunctions.json)

The following code in customfunctions.json specifies the metadata for the same ADD42 function.


Detailed reference information for the JSON file, including options not used in this example, is at Custom Functions Registration JSON.

Note that for this example:

  • There's only one custom function, so there's only one member of the functions array.
  • The name property defines the function name. As you see in the animated gif shown previously, a namespace (CONTOSO) is prepended to the function name in the Excel autocomplete menu. This prefix is defined in the add-in manifest, described below. The prefix and the function name are separated using a period, and by convention prefixes and function names are uppercase. To use your custom function, a user types the namespace followed by the function's name (ADD42) into a cell, in this case =CONTOSO.ADD42. The prefix is intended to be used as an identifier for your company or the add-in.
  • The description appears in the autocomplete menu in Excel.
  • When the user requests help for a function, Excel opens a task pane and displays the web page found at the URL specified in helpUrl.
  • The result property specifies the type of information returned by the function to Excel. The type child property can "string", "number", or "boolean". The dimensionality property can be scalar or matrix (a two-dimensional array of values of the specified type.)
  • The parameters array specifies, in order, the type of data in each parameter that is passed to the function. The name and description child properties are used in the Excel intellisense. The type and dimensionality child properties are identical to the child properties of the result property described above.
  • The options property enables you to customize some aspects of how and when Excel executes the function. There is more information about these options later in this article.

      "functions": [
              "name": "ADD42", 
              "description":  "adds 42 to the input numbers",
              "helpUrl": "",
              "result": {
                  "type": "number",
                  "dimensionality": "scalar"
              "parameters": [
                      "name": "number 1",
                      "description": "the first number to be added",
                      "type": "number",
                      "dimensionality": "scalar"
                      "name": "number 2",
                      "description": "the second number to be added",
                      "type": "number",
                      "dimensionality": "scalar"
              "options": {
                  "sync": true


The custom functions are registered when a user runs the add-in for the first time. After that, they are available, for that same user, in all workbooks (not only the one where the add-in ran initially.)

Your server settings for the JSON file must have CORS enabled in order for custom functions to work correctly in Excel Online.

Manifest file (customfunctions.xml)

The following is an example of the <ExtensionPoint> and <Resources> markup that you include in the add-in's manifest to enable Excel to run your functions. Note the following about this markup:

  • The <Script> element and its corresponding resource ID specifies the location of the JavaScript file with your functions.
  • The <Page> element and its corresponding resource ID specifies the location of the HTML page of your add-in. The HTML page includes a <Script> tag that loads the JavaScript file (customfunctions.js). The HTML page is a hidden page and is never displayed in the UI.
  • The <Metadata> element and its corresponding resource ID specifies the location of the JSON file.
  • A <Namespace> element and its corresponding resource ID specifies the prefix for all custom functions in the add-in.
<VersionOverrides xmlns="" xsi:type="VersionOverridesV1\_0">
        <Host xsi:type="Workbook">
                <ExtensionPoint xsi:type="CustomFunctions">
                        <SourceLocation resid="residjs" />
                        <SourceLocation resid="residhtml"/>
                        <SourceLocation resid="residjson" />
                    <Namespace resid="residNS" />
            <bt:Url id="residjson" DefaultValue="" />
            <bt:Url id="residjs" DefaultValue="" />
            <bt:Url id="residhtml" DefaultValue="" />
            <bt:String id="residNS" DefaultValue="CONTOSO" />

Initializing custom functions

Your code must initialize the custom functions feature before using it. You can do this either in a <Script> tag in the HTML file (customfunctions.html) or at the top of the JavaScript file (customfunctions.js). During the preview of custom functions, you have your choice of two syntaxes for intializing. The HTML file in the repo uses the following syntax:

Office.initialize = function (reason) {
    return Excel.CustomFunctions.initialize();

You can also use the following syntax:


Synchronous and asynchronous functions

The function ADD42 above is synchronous with respect to Excel (designated by setting the option "sync": true in the JSON file). Synchronous functions offer fast performance because they run in the same process as Excel and they run in parallel during multithreaded calculation.

On the other hand, if your custom function retrieves data from the web, it must be asynchronous with respect to Excel. Asynchronous functions must:

  1. Return a JavaScript Promise to Excel.
  2. Resolve the Promise with the final value using the callback function.

The following code shows an example of an asynchronous custom function that retrieves the temperature of a thermometer. Note that sendWebRequest is a hypothetical function, not specified here, that uses XHR to call a temperature web service.

function getTemperature(thermometerID){
    return new OfficeExtension.Promise(function(setResult){
        sendWebRequest(thermometerID, function(data){

Asynchronous functions display a GETTING_DATA temporary error in the cell while Excel waits for the final result. Users can interact normally with the rest of the spreadsheet while they wait for the result.


Custom functions are asynchronous by default. To designate functions as synchronous set the option "sync": true in the options property for the custom function in the registration JSON file.

Streamed functions

An asynchronous function can be streamed. Streamed custom functions let you output data to cells repeatedly over time, without waiting for Excel or users to request recalculations. The following example is a custom function that adds a number to the result every second. Note the following about this code:

  • Excel displays each new value automatically using the setResult callback.
  • The final parameter, caller, is never specified in your registration code, and it does not display in the autocomplete menu to Excel users when they enter the function. It’s an object that contains a setResult callback function that’s used to pass data from the function to Excel to update the value of a cell.
  • In order for Excel to pass the setResult function in the caller object, you must declare support for streaming during your function registration by setting the option "stream": true in the options property for the custom function in the registration JSON file.
function incrementValue(increment, caller){
    var result = 0;
         result += increment;
    }, 1000);


You can cancel streamed functions and asynchronous functions. Canceling your function calls is important to reduce their bandwith consumption, working memory, and CPU load. Excel cancels function calls in the following situations:

  • The user edits or deletes a cell that references the function.
  • One of the arguments (inputs) for the function changes. In this case, a new function call is triggered in addition to the cancelation.
  • The user triggers recalculation manually. As with the above case, a new function call is triggered in addition to the cancelation.

You must implement a cancellation handler for every streaming function. Asynchronous, non-streaming functions may or may not be cancelable; it's up to you. Synchronous functions cannot be canceled.

To make a function cancelable, set the option "cancelable": true in the options property for the custom function in the registration JSON file.

The following code shows the previous example with cancellation implemented. In the code, the caller object contains an onCanceled function must be defined for each cancelable custom function.

function incrementValue(increment, caller){ 
    var result = 0;
    var timer = setInterval(function(){
         result += increment;
    }, 1000);

    caller.onCanceled = function(){

Saving and sharing state

Asynchronous custom functions can save data in global JavaScript variables. In subsequent calls, your custom function may use the values saved in these variables. Saved state is useful when users add the same custom function to more than one cell, because all the instances of the function can share the state. For example, you may save the data returned from a call to a web resource to avoid making additional calls to the same web resource.

The following code shows an implementation of the previous temperature-streaming function that saves state globally. Note the following about this code:

  • refreshTemperature is a streamed function that reads the temperature of a particular thermometer every second. New temperatures are saved in the savedTemperatures variable, but does not directly update the cell value. It should not be directly called from a worksheet cell, so it is not registered in the JSON file.
  • streamTemperature updates the temperature values displayed in the cell every second and it uses savedTemperatures variable as its data source. It must be registered in the JSON file, and named with all upper-case letters, STREAMTEMPERATURE.
  • Users may call streamTemperature from several cells in the Excel UI. Each call reads data from the same savedTemperatures variable.
var savedTemperatures;

function streamTemperature(thermometerID, caller){ 
         refreshTemperatures(thermometerID); // starts fetching temperatures if the thermometer hasn't been read yet

     function getNextTemperature(){
         caller.setResult(savedTemperatures[thermometerID]); // setResult sends the saved temperature value to Excel.
         setTimeout(getNextTemperature, 1000); // Wait 1 second before updating Excel again.

function refreshTemperature(thermometerID){
     sendWebRequest(thermometerID, function(data){
         savedTemperatures[thermometerID] = data.temperature;
     }, 1000); // Wait 1 second before reading the thermometer again, and then update the saved temperature of thermometerID.


Synchronous functions (designated by setting the option "sync": true in the JSON file) cannot share state because Excel parallelizes them during multithreaded calculation. Only asynchronous functions may share state because an add-in's synchronous functions share the same JavaScript context in each session.

Working with ranges of data

Your custom function can take a range of data as a parameter, or you can return a range of data from a custom function.

For example, suppose that your function returns the second highest value from a range of numbers stored in Excel. The following function takes the parameter values, which is an Excel.CustomFunctionDimensionality.matrix parameter type. Note that in the registration JSON for this function, you would set the parameter's type property to matrix.

function secondHighest(values){ 
     var highest = values[0][0], secondHighest = values[0][0];
     for(var i = 0; i < values.length; i++){
         for(var j = 1; j < values[i].length; j++){
             if(values[i][j] >= highest){
                 secondHighest = highest;
                 highest = values[i][j];
             else if(values[i][j] >= secondHighest){
                 secondHighest = values[i][j];
     return secondHighest;

As you can see, ranges are handled in JavaScript as arrays of row arrays (like a 2-dimensional array).

Known issues

  • Help URLs and parameter descriptions are not yet used by Excel.
  • Custom functions are not currently available on Excel for mobile clients.
  • Currently, add-ins rely on a hidden browser process to run asynchronous custom functions. In the future, JavaScript will run directly on some platforms to ensure custom functions are faster and use less memory. Additionally, the HTML page referenced by the <Page> element in the manifest won’t be needed for most platforms because Excel will run the JavaScript directly. To prepare for this change, ensure your custom functions do not use the web page DOM. The supported host APIs for accessing the web will be WebSocket and XHR using GET or POST.
  • Volatile functions (those which recalculate automatically whenever unrelated data changes in the spreadsheet) are not yet supported.
  • Debugging is only enabled for asynchronous functions on Excel for Windows.
  • Deployment via the Office 365 Admin Portal and AppSource are not yet enabled.
  • Custom functions in Excel Online may stop working during a session after a period of inactivity. Refresh the browser page (F5) and re-enter a custom function to restore the feature.


  • Nov 7, 2017: Shipped the custom functions preview and samples
  • Nov 20, 2017: Fixed compatibility bug for those using builds 8801 and later
  • Nov 28, 2017: Shipped support for cancellation on asynchronous functions (requires change for streaming functions)
  • May 7, 2018: Shipped support for Mac, Excel Online, and synchronous functions running in-process