watch() once option

src/api/reactivity-core.md

@@ -361,6 +361,7 @@ Watches one or more reactive data sources and invokes a callback function when t
     flush?: 'pre' | 'post' | 'sync' // default: 'pre'
     onTrack?: (event: DebuggerEvent) => void
     onTrigger?: (event: DebuggerEvent) => void
+    once?: boolean // default: false (3.4+)
   }
   ```
 
@@ -387,6 +388,7 @@ Watches one or more reactive data sources and invokes a callback function when t
   - **`deep`**: force deep traversal of the source if it is an object, so that the callback fires on deep mutations. See [Deep Watchers](/guide/essentials/watchers#deep-watchers).
   - **`flush`**: adjust the callback's flush timing. See [Callback Flush Timing](/guide/essentials/watchers#callback-flush-timing) and [`watchEffect()`](/api/reactivity-core#watcheffect).
   - **`onTrack / onTrigger`**: debug the watcher's dependencies. See [Watcher Debugging](/guide/extras/reactivity-in-depth#watcher-debugging).
+  - **`once`**: run the callback only once. The watcher is automatically stopped after the first callback run. <sup class="vt-badge" data-text="3.4+" />
 
   Compared to [`watchEffect()`](#watcheffect), `watch()` allows us to: