office-js

The JavaScript API for Office enables cross-platform web add-ins for Excel, Word, PowerPoint, and Outlook.

Office-js is the unified JavaScript library (Office.js) used to build custom functionality into Microsoft 365 applications. It replaces legacy VBA and COM add-ins with modern web standards (HTML, CSS, and JS) that run across Windows, Mac, and web browsers. Developers use this API to manipulate objects like Excel ranges, Word paragraphs, and Outlook mail items through asynchronous calls. By leveraging the Office.js CDN, teams deploy updates instantly without requiring local installation on client machines.

https://learn.microsoft.com/en-us/office/dev/add-ins/reference/javascript-api-for-office