Reactivity API: Core
See also
To better understand the Reactivity APIs, it is recommended to read the following chapters in the guide:
- Reactivity Fundamentals (with the API preference set to Composition API)
- Reactivity in Depth
ref()
Takes an inner value and returns a reactive and mutable ref object, which has a single property .value
that points to the inner value.
Type
tsfunction ref<T>(value: T): Ref<UnwrapRef<T>> interface Ref<T> { value: T }
Details
The ref object is mutable - i.e. you can assign new values to
.value
. It is also reactive - i.e. any read operations to.value
are tracked, and write operations will trigger associated effects.If an object is assigned as a ref's value, the object is made deeply reactive with reactive(). This also means if the object contains nested refs, they will be deeply unwrapped.
To avoid the deep conversion, use
shallowRef()
instead.Example
jsconst count = ref(0) console.log(count.value) // 0 count.value = 1 console.log(count.value) // 1
See also
computed()
Takes a getter function and returns a readonly reactive ref object for the returned value from the getter. It can also take an object with get
and set
functions to create a writable ref object.
Type
ts// read-only function computed<T>( getter: (oldValue: T | undefined) => T, // see "Computed Debugging" link below debuggerOptions?: DebuggerOptions ): Readonly<Ref<Readonly<T>>> // writable function computed<T>( options: { get: (oldValue: T | undefined) => T set: (value: T) => void }, debuggerOptions?: DebuggerOptions ): Ref<T>
Example
Creating a readonly computed ref:
jsconst count = ref(1) const plusOne = computed(() => count.value + 1) console.log(plusOne.value) // 2 plusOne.value++ // error
Creating a writable computed ref:
jsconst count = ref(1) const plusOne = computed({ get: () => count.value + 1, set: (val) => { count.value = val - 1 } }) plusOne.value = 1 console.log(count.value) // 0
Debugging:
jsconst plusOne = computed(() => count.value + 1, { onTrack(e) { debugger }, onTrigger(e) { debugger } })
See also
reactive()
Returns a reactive proxy of the object.
Type
tsfunction reactive<T extends object>(target: T): UnwrapNestedRefs<T>
Details
The reactive conversion is "deep": it affects all nested properties. A reactive object also deeply unwraps any properties that are refs while maintaining reactivity.
It should also be noted that there is no ref unwrapping performed when the ref is accessed as an element of a reactive array or a native collection type like
Map
.To avoid the deep conversion and only retain reactivity at the root level, use shallowReactive() instead.
The returned object and its nested objects are wrapped with ES Proxy and not equal to the original objects. It is recommended to work exclusively with the reactive proxy and avoid relying on the original object.
Example
Creating a reactive object:
jsconst obj = reactive({ count: 0 }) obj.count++
Ref unwrapping:
tsconst count = ref(1) const obj = reactive({ count }) // ref will be unwrapped console.log(obj.count === count.value) // true // it will update `obj.count` count.value++ console.log(count.value) // 2 console.log(obj.count) // 2 // it will also update `count` ref obj.count++ console.log(obj.count) // 3 console.log(count.value) // 3
Note that refs are not unwrapped when accessed as array or collection elements:
jsconst books = reactive([ref('Vue 3 Guide')]) // need .value here console.log(books[0].value) const map = reactive(new Map([['count', ref(0)]])) // need .value here console.log(map.get('count').value)
When assigning a ref to a
reactive
property, that ref will also be automatically unwrapped:tsconst count = ref(1) const obj = reactive({}) obj.count = count console.log(obj.count) // 1 console.log(obj.count === count.value) // true
See also
readonly()
Takes an object (reactive or plain) or a ref and returns a readonly proxy to the original.
Type
tsfunction readonly<T extends object>( target: T ): DeepReadonly<UnwrapNestedRefs<T>>
Details
A readonly proxy is deep: any nested property accessed will be readonly as well. It also has the same ref-unwrapping behavior as
reactive()
, except the unwrapped values will also be made readonly.To avoid the deep conversion, use shallowReadonly() instead.
Example
jsconst original = reactive({ count: 0 }) const copy = readonly(original) watchEffect(() => { // works for reactivity tracking console.log(copy.count) }) // mutating original will trigger watchers relying on the copy original.count++ // mutating the copy will fail and result in a warning copy.count++ // warning!
watchEffect()
Runs a function immediately while reactively tracking its dependencies and re-runs it whenever the dependencies are changed.
Type
tsfunction watchEffect( effect: (onCleanup: OnCleanup) => void, options?: WatchEffectOptions ): WatchHandle type OnCleanup = (cleanupFn: () => void) => void interface WatchEffectOptions { flush?: 'pre' | 'post' | 'sync' // default: 'pre' onTrack?: (event: DebuggerEvent) => void onTrigger?: (event: DebuggerEvent) => void } interface WatchHandle { (): void // callable, same as `stop` pause: () => void resume: () => void stop: () => void }
Details
The first argument is the effect function to be run. The effect function receives a function that can be used to register a cleanup callback. The cleanup callback will be called right before the next time the effect is re-run, and can be used to clean up invalidated side effects, e.g. a pending async request (see example below).
The second argument is an optional options object that can be used to adjust the effect's flush timing or to debug the effect's dependencies.
By default, watchers will run just prior to component rendering. Setting
flush: 'post'
will defer the watcher until after component rendering. See Callback Flush Timing for more information. In rare cases, it might be necessary to trigger a watcher immediately when a reactive dependency changes, e.g. to invalidate a cache. This can be achieved usingflush: 'sync'
. However, this setting should be used with caution, as it can lead to problems with performance and data consistency if multiple properties are being updated at the same time.The return value is a handle function that can be called to stop the effect from running again.
Example
jsconst count = ref(0) watchEffect(() => console.log(count.value)) // -> logs 0 count.value++ // -> logs 1
Stopping the watcher:
jsconst stop = watchEffect(() => {}) // when the watcher is no longer needed: stop()
Pausing / resuming the watcher:
jsconst { stop, pause, resume } = watchEffect(() => {}) // temporarily pause the watcher pause() // resume later resume() // stop stop()
Side effect cleanup:
jswatchEffect(async (onCleanup) => { const { response, cancel } = doAsyncWork(newId) // `cancel` will be called if `id` changes, cancelling // the previous request if it hasn't completed yet onCleanup(cancel) data.value = await response })
Side effect cleanup in 3.5+:
jsimport { onWatcherCleanup } from 'vue' watchEffect(async () => { const { response, cancel } = doAsyncWork(newId) // `cancel` will be called if `id` changes, cancelling // the previous request if it hasn't completed yet onWatcherCleanup(cancel) data.value = await response })
Options:
jswatchEffect(() => {}, { flush: 'post', onTrack(e) { debugger }, onTrigger(e) { debugger } })
See also
watchPostEffect()
Alias of watchEffect()
with flush: 'post'
option.
watchSyncEffect()
Alias of watchEffect()
with flush: 'sync'
option.
watch()
Watches one or more reactive data sources and invokes a callback function when the sources change.
Type
ts// watching single source function watch<T>( source: WatchSource<T>, callback: WatchCallback<T>, options?: WatchOptions ): WatchHandle // watching multiple sources function watch<T>( sources: WatchSource<T>[], callback: WatchCallback<T[]>, options?: WatchOptions ): WatchHandle type WatchCallback<T> = ( value: T, oldValue: T, onCleanup: (cleanupFn: () => void) => void ) => void type WatchSource<T> = | Ref<T> // ref | (() => T) // getter | (T extends object ? T : never) // reactive object interface WatchOptions extends WatchEffectOptions { immediate?: boolean // default: false deep?: boolean | number // default: false flush?: 'pre' | 'post' | 'sync' // default: 'pre' onTrack?: (event: DebuggerEvent) => void onTrigger?: (event: DebuggerEvent) => void once?: boolean // default: false (3.4+) } interface WatchHandle { (): void // callable, same as `stop` pause: () => void resume: () => void stop: () => void }
Types are simplified for readability.
Details
watch()
is lazy by default - i.e. the callback is only called when the watched source has changed.The first argument is the watcher's source. The source can be one of the following:
- A getter function that returns a value
- A ref
- A reactive object
- ...or an array of the above.
The second argument is the callback that will be called when the source changes. The callback receives three arguments: the new value, the old value, and a function for registering a side effect cleanup callback. The cleanup callback will be called right before the next time the effect is re-run, and can be used to clean up invalidated side effects, e.g. a pending async request.
When watching multiple sources, the callback receives two arrays containing new / old values corresponding to the source array.
The third optional argument is an options object that supports the following options:
immediate
: trigger the callback immediately on watcher creation. Old value will beundefined
on the first call.deep
: force deep traversal of the source if it is an object, so that the callback fires on deep mutations. In 3.5+, this can also be a number indicating the max traversal depth. See Deep Watchers.flush
: adjust the callback's flush timing. See Callback Flush Timing andwatchEffect()
.onTrack / onTrigger
: debug the watcher's dependencies. See Watcher Debugging.once
: (3.4+) run the callback only once. The watcher is automatically stopped after the first callback run.
Compared to
watchEffect()
,watch()
allows us to:- Perform the side effect lazily;
- Be more specific about what state should trigger the watcher to re-run;
- Access both the previous and current value of the watched state.
Example
Watching a getter:
jsconst state = reactive({ count: 0 }) watch( () => state.count, (count, prevCount) => { /* ... */ } )
Watching a ref:
jsconst count = ref(0) watch(count, (count, prevCount) => { /* ... */ })
When watching multiple sources, the callback receives arrays containing new / old values corresponding to the source array:
jswatch([fooRef, barRef], ([foo, bar], [prevFoo, prevBar]) => { /* ... */ })
When using a getter source, the watcher only fires if the getter's return value has changed. If you want the callback to fire even on deep mutations, you need to explicitly force the watcher into deep mode with
{ deep: true }
. Note in deep mode, the new value and the old will be the same object if the callback was triggered by a deep mutation:jsconst state = reactive({ count: 0 }) watch( () => state, (newValue, oldValue) => { // newValue === oldValue }, { deep: true } )
When directly watching a reactive object, the watcher is automatically in deep mode:
jsconst state = reactive({ count: 0 }) watch(state, () => { /* triggers on deep mutation to state */ })
watch()
shares the same flush timing and debugging options withwatchEffect()
:jswatch(source, callback, { flush: 'post', onTrack(e) { debugger }, onTrigger(e) { debugger } })
Stopping the watcher:
jsconst stop = watch(source, callback) // when the watcher is no longer needed: stop()
Pausing / resuming the watcher:
jsconst { stop, pause, resume } = watch(() => {}) // temporarily pause the watcher pause() // resume later resume() // stop stop()
Side effect cleanup:
jswatch(id, async (newId, oldId, onCleanup) => { const { response, cancel } = doAsyncWork(newId) // `cancel` will be called if `id` changes, cancelling // the previous request if it hasn't completed yet onCleanup(cancel) data.value = await response })
Side effect cleanup in 3.5+:
jsimport { onWatcherCleanup } from 'vue' watch(id, async (newId) => { const { response, cancel } = doAsyncWork(newId) onWatcherCleanup(cancel) data.value = await response })
See also
onWatcherCleanup()
Register a cleanup function to be executed when the current watcher is about to re-run. Can only be called during the synchronous execution of a watchEffect
effect function or watch
callback function (i.e. it cannot be called after an await
statement in an async function.)
Type
tsfunction onWatcherCleanup( cleanupFn: () => void, failSilently?: boolean ): void
Example
tsimport { watch, onWatcherCleanup } from 'vue' watch(id, (newId) => { const { response, cancel } = doAsyncWork(newId) // `cancel` will be called if `id` changes, cancelling // the previous request if it hasn't completed yet onWatcherCleanup(cancel) })