Composition API: Dependency Injection | Vue.js

Composition API:
Dependency Injection
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provide()
​

Provides a value that can be injected by descendant components.

Type

ts
function provide<T>(key: InjectionKey<T> | string, value: T): void

Details

provide() takes two arguments: the key, which can be a string or a symbol, and the value to be injected.

When using TypeScript, the key can be a symbol casted as InjectionKey - a Vue provided utility type that extends Symbol, which can be used to sync the value type between provide() and inject().

Similar to lifecycle hook registration APIs, provide() must be called synchronously during a component's setup() phase.

Example

vue
<script setup>
import { ref, provide } from 'vue'
import { fooSymbol } from './injectionSymbols'

// provide static value
provide('foo', 'bar')

// provide reactive value
const count = ref(0)
provide('count', count)

// provide with Symbol keys
provide(fooSymbol, count)
</script>

See also

Guide - Provide / Inject
Guide - Typing Provide / Inject 
inject()
​

Injects a value provided by an ancestor component or the application (via app.provide()).

Type

ts
// without default value
function inject<T>(key: InjectionKey<T> | string): T | undefined

// with default value
function inject<T>(key: InjectionKey<T> | string, defaultValue: T): T

// with factory
function inject<T>(
  key: InjectionKey<T> | string,
  defaultValue: () => T,
  treatDefaultAsFactory: true
): T

Details

The first argument is the injection key. Vue will walk up the parent chain to locate a provided value with a matching key. If multiple components in the parent chain provides the same key, the one closest to the injecting component will "shadow" those higher up the chain. If no value with matching key was found, inject() returns undefined unless a default value is provided.

The second argument is optional and is the default value to be used when no matching value was found.

The second argument can also be a factory function that returns values that are expensive to create. In this case, true must be passed as the third argument to indicate that the function should be used as a factory instead of the value itself.

Similar to lifecycle hook registration APIs, inject() must be called synchronously during a component's setup() phase.

When using TypeScript, the key can be of type of InjectionKey - a Vue-provided utility type that extends Symbol, which can be used to sync the value type between provide() and inject().

Example

Assuming a parent component has provided values as shown in the previous provide() example:

vue
<script setup>
import { inject } from 'vue'
import { fooSymbol } from './injectionSymbols'

// inject static value without default
const foo = inject('foo')

// inject reactive value
const count = inject('count')

// inject with Symbol keys
const foo2 = inject(fooSymbol)

// inject with default value
const bar = inject('foo', 'default value')

// inject with function default value
const fn = inject('function', () => {})

// inject with default value factory
const baz = inject('factory', () => new ExpensiveObject(), true)
</script>

See also

Guide - Provide / Inject
Guide - Typing Provide / Inject