Reactivity API: Core | Vue.js

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

ts
function 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

js
const count = ref(0)
console.log(count.value) // 0

count.value = 1
console.log(count.value) // 1

See also

Guide - Reactivity Fundamentals with ref()
Guide - Typing ref() 
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: () => T,
  // see "Computed Debugging" link below
  debuggerOptions?: DebuggerOptions
): Readonly<Ref<Readonly<T>>>

// writable
function computed<T>(
  options: {
    get: () => T
    set: (value: T) => void
  },
  debuggerOptions?: DebuggerOptions
): Ref<T>

Example

Creating a readonly computed ref:

js
const count = ref(1)
const plusOne = computed(() => count.value + 1)

console.log(plusOne.value) // 2

plusOne.value++ // error

Creating a writable computed ref:

js
const 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:

js
const plusOne = computed(() => count.value + 1, {
  onTrack(e) {
    debugger
  },
  onTrigger(e) {
    debugger
  }
})

See also

Guide - Computed Properties
Guide - Computed Debugging
Guide - Typing computed() 
reactive()
​

Returns a reactive proxy of the object.

Type

ts
function 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:

js
const obj = reactive({ count: 0 })
obj.count++

Ref unwrapping:

ts
const 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:

js
const 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:

ts
const count = ref(1)
const obj = reactive({})

obj.count = count

console.log(obj.count) // 1
console.log(obj.count === count.value) // true

See also

Guide - Reactivity Fundamentals
Guide - Typing reactive() 
readonly()
​

Takes an object (reactive or plain) or a ref and returns a readonly proxy to the original.

Type

ts
function 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

js
const 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

ts
function watchEffect(
  effect: (onCleanup: OnCleanup) => void,
  options?: WatchEffectOptions
): StopHandle

type OnCleanup = (cleanupFn: () => void) => void

interface WatchEffectOptions {
  flush?: 'pre' | 'post' | 'sync' // default: 'pre'
  onTrack?: (event: DebuggerEvent) => void
  onTrigger?: (event: DebuggerEvent) => void
}

type StopHandle = () => 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 using flush: '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

js
const count = ref(0)

watchEffect(() => console.log(count.value))
// -> logs 0

count.value++
// -> logs 1

Side effect cleanup:

js
watchEffect(async (onCleanup) => {
  const { response, cancel } = doAsyncWork(id.value)
  // `cancel` will be called if `id` changes
  // so that previous pending request will be cancelled
  // if not yet completed
  onCleanup(cancel)
  data.value = await response
})

Stopping the watcher:

js
const stop = watchEffect(() => {})

// when the watcher is no longer needed:
stop()

Options:

js
watchEffect(() => {}, {
  flush: 'post',
  onTrack(e) {
    debugger
  },
  onTrigger(e) {
    debugger
  }
})

See also

Guide - Watchers
Guide - Watcher Debugging
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
): StopHandle

// watching multiple sources
function watch<T>(
  sources: WatchSource<T>[],
  callback: WatchCallback<T[]>,
  options?: WatchOptions
): StopHandle

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 // default: false
  flush?: 'pre' | 'post' | 'sync' // default: 'pre'
  onTrack?: (event: DebuggerEvent) => void
  onTrigger?: (event: DebuggerEvent) => 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 be undefined on the first call.
deep: force deep traversal of the source if it is an object, so that the callback fires on deep mutations. See Deep Watchers.
flush: adjust the callback's flush timing. See Callback Flush Timing and watchEffect().
onTrack / onTrigger: debug the watcher's dependencies. See Watcher Debugging.

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:

js
const state = reactive({ count: 0 })
watch(
  () => state.count,
  (count, prevCount) => {
    /* ... */
  }
)

Watching a ref:

js
const count = ref(0)
watch(count, (count, prevCount) => {
  /* ... */
})

When watching multiple sources, the callback receives arrays containing new / old values corresponding to the source array:

js
watch([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:

js
const 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:

js
const state = reactive({ count: 0 })
watch(state, () => {
  /* triggers on deep mutation to state */
})

watch() shares the same flush timing and debugging options with watchEffect():

js
watch(source, callback, {
  flush: 'post',
  onTrack(e) {
    debugger
  },
  onTrigger(e) {
    debugger
  }
})

Stopping the watcher:

js
const stop = watch(source, callback)

// when the watcher is no longer needed:
stop()

Side effect cleanup:

js
watch(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
})

See also

Guide - Watchers
Guide - Watcher Debugging