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Issue 44584 Clarify filtering comments in background scripts page #44592

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Issue 44584 Clarify filtering comments in background scripts page #44592

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6 changes: 5 additions & 1 deletion files/en-us/mozilla/add-ons/webextensions/background_scripts/index.md
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Expand Up @@ -164,7 +164,9 @@ browser.runtime.onMessage.addListener(

### Filter events

Use APIs that support event filters to restrict listeners to the cases the extension cares about. If an extension is listening for {{WebExtAPIRef("tabs.onUpdated")}}, use the {{WebExtAPIRef("webNavigation.onCompleted")}} event with filters instead, as the tabs API does not support filters.
When your extension only needs to act on a subset of events, such as when a web page is opened from a specific domain, use event filters where they are available.

For example, you can add a filter to the {{WebExtAPIRef("webNavigation.onCompleted")}} event to start your background script when certain URLs load, like this:

```js
browser.webNavigation.onCompleted.addListener(
Expand All @@ -175,6 +177,8 @@ browser.webNavigation.onCompleted.addListener(
);
```

When targeting Firefox desktop only, you can also add filters to {{WebExtAPIRef("tabs.onUpdated")}} to achieve a similar outcome.

### React to listeners

Listeners exist to trigger functionality once an event has fired. To react to an event, structure the desired reaction inside the listener event.
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