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Excel JavaScript API

Office 2016 introduced a JavaScript API for interacting with the application. It offers solutions for custom functions as well as task panes.

Excel currently does not provide support for working with Apple Numbers files and some legacy file formats. SheetJS fills the gap.

This demo creates a new custom function to add much-needed functionality:

SHEETJS.EXTERN() tries to fetch an external spreadsheet and insert the data into the worksheet.

This demo focuses on the basic mechanics. Advanced topics like Excel Custom Function parameters are covered in the official Office JavaScript API docs. SheetJS worksheet metadata and other properties are covered in this doc site.

Creating a new Add-in

Initial Platform Setup (click to show)

The tool for generating Office Add-ins depends on NodeJS and various libraries. Install NodeJS and the required dependencies:

npm install -g yo bower generator-office

Creating a new Project (click to show)

Run yo office from the command line. It will ask a few questions.

"Choose a project type": "Excel Custom Functions Add-in project"
"Choose a script type": "JavaScript",
"What do you want to name your add-in?": "SheetJSImport"

You will see a screen like

? Choose a project type: Excel Custom Functions Add-in project
? Choose a script type: JavaScript
? What do you want to name your add-in? SheetJSImport

----------------------------------------------------------------------------------

      Creating SheetJSImport add-in for Excel using JavaScript and Excel-functions
at C:\Users\SheetJS\Documents\SheetJSImport

----------------------------------------------------------------------------------

It helpfully prints out the next steps:

cd SheetJSImport
npm run build
npm start

If you have VSCodium installed, the folder can be opened with

codium .

Running npm start will open up a terminal window and a new Excel window with the loaded add-in. Keep the terminal window open (it can be minimized). When you make a change, close both the Excel window and the terminal window before running npm start again.

Integrating the SheetJS Library

The library can be installed like any other NodeJS module:

npm i --save https://cdn.sheetjs.com/xlsx-latest/xlsx-latest.tgz

To be sure the library is loaded, remove all of the existing functions from src\functions\functions.js. The new contents should be

var XLSX = require("xlsx");

/**
 * Print SheetJS Library Version
 * @customfunction
 * @returns {string[][]} The SheetJS Library Version.
 */
function version() {
  return [[XLSX.version]];
}

The manifest.xml should also be updated to reflect the function namespace:

        <bt:String id="Functions.Namespace" DefaultValue="SHEETJS"/>

After making the change, save the files. Close the terminal window and the Excel window (do not save the Excel file). Re-run npm start.

In the new Excel window, enter the formula =SHEETJS.VERSION() in cell E1. You should see something similar to the following screenshot:

This indicates that the SheetJS library has been loaded.

Dynamic Arrays and SheetJS Array of Arrays

The sheet_to_json helper function can generate arrays of arrays of values based on the worksheet data. Excel custom functions transparently treat these as Dynamic Arrays.

Fetching Files from the Internet

For the next step, we will try to fetch data from an external resource. https://sheetjs.com/pres.numbers is an Apple Numbers file. Excel does not understand Numbers files and it will not open them.

Excel bug related to `fetch` (click to show)

fetch is available to custom functions:

async function extern() {
  try {
    const url = "https://sheetjs.com/pres.numbers"; // URL to download
    const res = await fetch(url); // fetch data
    const ab = await res.arrayBuffer(); // get data as an array buffer

    // DO SOMETHING WITH THE DATA HERE

  } catch(e) { return e; } // pass error back to Excel
}

When fetching data, functions typically receive an ArrayBuffer which stores the file data. This is readily parsed with read:

var wb = XLSX.read(ab); // parse workbook

This is how it should work.

There are outstanding bugs

For the purposes of this demo, a Base64-encoded file will be used. The workaround involves fetching that Base64 file, getting the text, and parsing with the base64 type:

async function extern() {
  try {
    const url = "https://sheetjs.com/pres.numbers.b64"; // URL to download
    const res = await fetch(url); // fetch data
    const text = await res.text(); // get data as an array buffer

    var wb = XLSX.read(text, { type: "base64" });
    // DO SOMETHING WITH THE DATA HERE

  } catch(e) { return e; } // pass error back to Excel
}

Base64-encoded files can be generated with PowerShell:

[convert]::ToBase64String([System.IO.File]::ReadAllBytes((Resolve-Path "path\to\file"))) > file.b64

The .Sheets property of the workbook object holds all of the worksheets and the .SheetNames property is an array of worksheet names. Picking the first worksheet is fairly straightforward:

var ws = wb.Sheets[wb.SheetNames[0]]; // get first worksheet

This data can be converted to an Array of Arrays in one line:

var aoa = XLSX.utils.sheet_to_json(ws, {header: 1}); // get data as array of arrays

To demonstrate the parsing ability, a Base64-encoded version of the file will be used. This file contains no binary characters and should "just work". Once the aforementioned Excel bug is fixed, the non-Base64 version can be used.

This new function should be added to src\functions\functions.js:

/**
 * Download file and write data
 * @customfunction
 * @returns {any[][]} Worksheet data
 */
async function extern() {
  try {
    /* URL */
    // const url = "https://sheetjs.com/pres.numbers"; // Once Excel bug is fixed
    const url = "https://sheetjs.com/pres.numbers.b64"; // workaround

    /* Fetch Data */
    const res = await fetch(url);

    /* Get Data */
    // const ab = await res.arrayBuffer(); // Once Excel bug is fixed
    const b64 = await res.text(); // workaround

    /* Parse Data */
    // var wb = XLSX.read(ab); // Once Excel bug is fixed
    var wb = XLSX.read(b64, { type: "base64" }); // workaround

    /* get and return data */
    var ws = wb.Sheets[wb.SheetNames[0]]; // get first worksheet
    var aoa = XLSX.utils.sheet_to_json(ws, { header: 1 }); // get data as array of arrays
    return [[url]];
  } catch(e) { return [[e]]; } // pass error back to Excel
}