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For a normal Chrome extension, there is no documented, general-purpose API to inject code into Slack’s service worker and intercept its background push notifications. Overriding window.Notification can catch notifications created in a page’s JavaScript context, but it cannot reach a separate service worker. That makes interception through supported extension APIs impractical—not a universal claim that no privileged debugging technique could ever manipulate a browser.
Why can’t the extension reach the service worker?
A page and its service worker are separate JavaScript execution contexts. Page code can access window; a service worker does not have the page’s window object. As a result, replacing the page’s window.Notification does not replace notification code running in the worker.
M. Bilal Khan’s account of investigating this problem reports that Slack’s background notifications use a service-worker push path: the worker handles a push event and calls its registration’s showNotification(). In that path, a page-world override is not reached. This is the author’s report, not a verified statement about every Slack version, browser, operating system, or notification configuration. Read the account.
Can I override window.Notification?
Yes, for calls made in the page’s JavaScript environment. If a page script creates a notification with new Notification(...), an override in that same context may observe or affect that call. It does not provide coverage for notifications produced by a service worker using registration.showNotification(). The distinction is about where the notification-producing code runs, not merely which website displays it.
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Why don’t Chrome’s extension APIs solve this?
Push API: receives pushes for your extension
Chrome’s Push API lets an extension subscribe to a push provider and receive messages in its own service worker, including waking that worker when a push arrives. It does not subscribe the extension to Slack’s push stream or grant access to Slack’s service worker. Chrome’s real-time extension guide describes the extension’s own messaging path.
chrome.webRequest: network events are not worker injection
The webRequest API exposes specified request lifecycle events and, where permitted, can support actions such as blocking or changing requests and headers. It is not a general-purpose way to execute code inside a third-party service worker or rewrite that worker’s script response. In Manifest V3, webRequestBlocking is unavailable to most extensions. Chrome also documents that the API can observe a WebSocket handshake but not individual messages after the connection is established. See Chrome’s webRequest reference.
These are separate capabilities: seeing a request, changing a permitted request, altering a response body, and running code in a service worker are not interchangeable. A visible network request does not, by itself, reveal application messages or let an extension take over the worker’s notification call.
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Debugger access: a different, disruptive route
Khan discusses debugger and Chrome DevTools Protocol manipulation as a technically possible avenue, but judges it unsuitable for a consumer extension because of the debugger warning and user experience. That is the author’s assessment, not a Chrome rule that CDP manipulation is impossible. It also does not create a supported, seamless interception hook for ordinary extension use.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What can you do instead?
React to events your extension owns
If your own service needs to notify your extension, use Chrome’s extension Push API and handle incoming messages in the extension’s service worker. For frequent two-way communication, a WebSocket may be appropriate while the extension is active; a connection alone does not provide an external wake-up event if the extension has been suspended. Choose this route when you control the producer or can arrange for it to send messages to your extension—not as a way to tap Slack’s browser notifications.
Build a separate, authorized Slack integration
If the actual goal is to respond to Slack workspace activity, build an app against Slack’s platform interfaces and obtain the required authorization. Slack’s Web API supports querying workspace information and taking actions; choose the relevant event or API mechanism and check its current scopes for your use case. This is a separate integration, not an API for intercepting notifications already generated by Slack in a browser. Slack Web API documentation.
Quick Recap
Choose the approach that matches the goal
| Approach | Where it runs | What it can do | Fit for intercepting Slack browser alerts |
|---|---|---|---|
Override window.Notification |
Page JavaScript | May observe notifications created directly by page code | Does not reach a separate service worker’s notification call |
| Chrome extension Push API | Your extension’s service worker | Receives pushes addressed to the extension | Does not subscribe to Slack’s push stream |
chrome.webRequest |
Extension network-request events | Observes specified requests and supports limited permitted actions | Does not provide arbitrary worker injection or general response-body rewriting |
| Slack app using platform APIs | Slack integration and its authorized backend | Works with workspace data and actions allowed by the chosen interfaces and scopes | A separate route to workspace events, not browser-notification interception |
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