Office.js Add-in
Also known as: Office Web Add-in, Outlook Web Add-in
An Office.js add-in is a web-tech (HTML/JS/CSS) extension for Outlook and the rest of Microsoft Office that runs cross-platform across Outlook Desktop (Windows and Mac), Outlook on the Web, and New Outlook. Office.js replaced the legacy VSTO/COM add-in model. it is the only extension model Microsoft supports for New Outlook and is the standard for any modern Outlook tracker in 2026.
Microsoft has been migrating Outlook off the legacy COM and VSTO add-in models for years. The endgame is New Outlook, which is built on the web stack and supports only Office.js add-ins. Legacy COM add-ins simply do not load in New Outlook.
Why Office.js matters in 2026
Any Outlook tracker that has not migrated to Office.js will lose functionality every time a customer adopts New Outlook. By the end of 2026, New Outlook is expected to be the default for most Microsoft 365 commercial users. A COM-only tracker will be invisible to those users.
How it works mechanically
Office.js add-ins are packaged as a manifest XML (or the newer JSON manifest) plus a hosted web app. The manifest declares which Outlook surfaces the add-in attaches to (compose, read, ribbon, taskpane, message extensions). The web app runs in a sandboxed iframe inside Outlook and communicates with the host via the Office JavaScript API.
What this enables
- Cross-platform: same code runs on Outlook Desktop Windows, Mac, Web, and New Outlook
- AppSource distribution: Microsoft's official channel for centrally-deployed enterprise rollouts
- Web stack: React, Vue, Svelte all work; no C# or VSTO knowledge required
- Modern auth: standard OAuth 2.0 flows
Related reading
Frequently asked questions
Is Outsolvi an Office.js add-in?+
Yes. Outsolvi for Outlook is a Microsoft 365-validated Office.js add-in distributed via AppSource, with feature parity across Outlook Desktop (Windows/Mac), Outlook on the Web, and New Outlook.
Are COM add-ins still supported?+
Classic Outlook Desktop still loads COM add-ins for now, but Microsoft has indicated long-term deprecation. New Outlook does not load them at all.
Put this concept into practice
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We update these pages when the underlying mechanics change. new mailbox-provider rules, new tracker behavior, new measurement gaps. The dates above are real revisions, not auto-touches.