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.
See how this shows up in your own inbox: try Outsolvi free for 14 days, no credit card.
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. Microsoft plans to make New Outlook the default for enterprise users from March 2027, with a way back to classic until a later cutover. 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 add-in store, one way vendors offer their add-ins (Outsolvi isn't listed there; you add it from a file or your admin deploys it)
- Web stack: React, Vue, Svelte all work; no C# or VSTO knowledge required
- Modern auth: standard OAuth 2.0 flows
Track real engagement, not Apple MPP noise.
Start Free TrialFrequently asked questions
Is Outsolvi an Office.js add-in?+
Yes, for Outlook on the web and the new Outlook for Windows. Outsolvi for Outlook is an Office add-in you add from your Outsolvi dashboard (it isn't in Microsoft's add-in store), and it works with Microsoft 365 work or school accounts. Classic Outlook for Windows uses a separate desktop add-in. Outlook for Mac doesn't track, so on a Mac, send from Outlook on the web.
Are COM add-ins still supported?+
Classic Outlook Desktop still loads COM add-ins. Microsoft says existing classic Outlook installs will be supported until at least 2029, and it has not announced a date to turn classic off. New Outlook does not load COM add-ins at all.
Put this concept into practice
Free tools, articles, and features on this same topic.
Want accurate tracking that handles Office.js Add-in?
Outsolvi tracks Outlook and Gmail, grades every open as a person or a machine, and flags hot leads, from $7/user/mo billed yearly. 14-day free trial, no credit card.
Start 14-Day Free TrialNate built Outsolvi after watching every email-tracking tool he had ever used lie to him about opens. Outsolvi grades every open as a person or a machine, in Outlook and Gmail, so the number on the dashboard is one a rep can actually act on.
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.