Reactivity API: Advanced

shallowRef()

Shallow version of ref().

• Type

  function shallowRef<T>(value: T): ShallowRef<T>
  
  interface ShallowRef<T> {
    value: T
  }

• Details

  Unlike ref(), the inner value of a shallow ref is stored and exposed as-is, and will not be made deeply reactive. Only the .value access is reactive.

  shallowRef() is typically used for performance optimizations of large data structures, or integration with external state management systems.

• Example

  const state = shallowRef({ count: 1 })
  
  // does NOT trigger change
  state.value.count = 2
  
  // does trigger change
  state.value = { count: 2 }

triggerRef()

Force trigger effects that depends on a shallow ref. This is typically used after making deep mutations to the inner value of a shallow ref.

• Type

  function triggerRef(ref: ShallowRef): void

• Example

  const shallow = shallowRef({
    greet: 'Hello, world'
  })
  
  // Logs "Hello, world" once for the first run-through
  watchEffect(() => {
    console.log(shallow.value.greet)
  })
  
  // This won't trigger the effect because the ref is shallow
  shallow.value.greet = 'Hello, universe'
  
  // Logs "Hello, universe"
  triggerRef(shallow)

customRef()

Creates a customized ref with explicit control over its dependency tracking and updates triggering.

• Type

  function customRef<T>(factory: CustomRefFactory<T>): Ref<T>
  
  type CustomRefFactory<T> = (
    track: () => void,
    trigger: () => void
  ) => {
    get: () => T
    set: (value: T) => void
  }

• Details

  customRef() expects a factory function, which receives track and trigger functions as arguments and should return an object with get and set methods.

  In general, track() should be called inside get(), and trigger() should be called inside set(). However, you have full control over when they should be called, or whether they should be called at all.

• Example

  Creating a debounced ref that only updates the value after a certain timeout after the latest set call:

  export function useDebouncedRef(value, delay = 200) {
    let timeout
    return customRef((track, trigger) => {
      return {
        get() {
          track()
          return value
        },
        set(newValue) {
          clearTimeout(timeout)
          timeout = setTimeout(() => {
            value = newValue
            trigger()
          }, delay)
        }
      }
    })
  }

shallowReactive()

Shallow version of reactive().

• Type

  function shallowReactive<T extends object>(target: T): T

• Details

  Unlike reactive(), there is no deep conversion: only root-level properties are reactive for a shallow reactive object. Property values are stored and exposed as-is - this also means properties with ref values will not be automatically unwrapped.

  Use with Caution

  Shallow data structures should only be used for root level state in a component. Avoid nesting it inside a deep reactive object as it creates a tree with inconsistent reactivity behavior which can be difficult to understand and debug.

• Example

  const state = shallowReactive({
    foo: 1,
    nested: {
      bar: 2
    }
  })
  
  // mutating state's own properties is reactive
  state.foo++
  
  // ...but does not convert nested objects
  isReactive(state.nested) // false
  
  // NOT reactive
  state.nested.bar++

shallowReadonly()

Shallow version of readonly().

• Type

  function shallowReadonly<T extends object>(target: T): Readonly<T>

• Details

  Unlike readonly(), there is no deep conversion: only root-level properties are made readonly. Property values are stored and exposed as-is - this also means properties with ref values will not be automatically unwrapped.

  Use with Caution

  Shallow data structures should only be used for root level state in a component. Avoid nesting it inside a deep reactive object as it creates a tree with inconsistent reactivity behavior which can be difficult to understand and debug.

• Example

  const state = shallowReadonly({
    foo: 1,
    nested: {
      bar: 2
    }
  })
  
  // mutating state's own properties will fail
  state.foo++
  
  // ...but works on nested objects
  isReadonly(state.nested) // false
  
  // works
  state.nested.bar++

toRaw()

Returns the raw, original object of a reactivity-created proxy.

• Type

  function toRaw<T>(proxy: T): T

• Details

  toRaw() can return the original object from proxies created by reactive(), readonly(), shallowReactive() or shallowReadonly().

  This is an escape hatch that can be used to temporarily read without incurring proxy access / tracking overhead or write without triggering changes. It is not recommended to hold a persistent reference to the original object. Use with caution.

• Example

  const foo = {}
  const reactiveFoo = reactive(foo)
  
  console.log(toRaw(reactiveFoo) === foo) // true

markRaw()

Marks an object so that it will never be converted to a proxy. Returns the object itself.

• Type

  function markRaw<T extends object>(value: T): T

• Example

  const foo = markRaw({})
  console.log(isReactive(reactive(foo))) // false
  
  // also works when nested inside other reactive objects
  const bar = reactive({ foo })
  console.log(isReactive(bar.foo)) // false

  Use with Caution

  markRaw() and shallow APIs such as shallowReactive() allow you to selectively opt-out of the default deep reactive/readonly conversion and embed raw, non-proxied objects in your state graph. They can be used for various reasons:

  • Some values simply should not be made reactive, for example a complex 3rd party class instance.

  • Skipping proxy conversion can provide performance improvements when rendering large lists with immutable data sources.

  They are considered advanced because the raw opt-out is only at the root level, so if you set a nested, non-marked raw object into a reactive object and then access it again, you get the proxied version back. This can lead to identity hazards - i.e. performing an operation that relies on object identity but using both the raw and the proxied version of the same object:

  const foo = markRaw({
    nested: {}
  })
  
  const bar = reactive({
    // although `foo` is marked as raw, foo.nested is not.
    nested: foo.nested
  })
  
  console.log(foo.nested === bar.nested) // false

  Identity hazards are in general rare. However, to properly utilize these APIs while safely avoiding identity hazards requires a solid understanding of how the reactivity system works.

effectScope()

Creates an effect scope object which can capture the reactive effects (i.e. computed and watchers) created within it so that these effects can be disposed together. For detailed use cases of this API, please consult its corresponding RFC.

• Type

  function effectScope(detached?: boolean): EffectScope
  
  interface EffectScope {
    run<T>(fn: () => T): T | undefined // undefined if scope is inactive
    stop(): void
  }

• Example

  const scope = effectScope()
  
  scope.run(() => {
    const doubled = computed(() => counter.value * 2)
  
    watch(doubled, () => console.log(doubled.value))
  
    watchEffect(() => console.log('Count: ', doubled.value))
  })
  
  // to dispose all effects in the scope
  scope.stop()

getCurrentScope()

Returns the current active effect scope if there is one.

• Type

  function getCurrentScope(): EffectScope | undefined

onScopeDispose()

Registers a dispose callback on the current active effect scope. The callback will be invoked when the associated effect scope is stopped.

This method can be used as a non-component-coupled replacement of onUnmounted in reusable composition functions, since each Reactive component's setup() function is also invoked in an effect scope.

• Type

  function onScopeDispose(fn: () => void): void