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.
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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
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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.
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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.