Composition

mixins

• Type: Array<Object>

• Details:

  The mixins option accepts an array of mixin objects. These mixin objects can contain instance options like normal instance objects, and they will be merged against the eventual options using the certain option merging logic. For example, if your mixin contains a created hook and the component itself also has one, both functions will be called.

  Mixin hooks are called in the order they are provided, and called before the component's own hooks.

  INFO

  In Vue 2, mixins were the primary mechanism for creating reusable chunks of component logic. While mixins continue to be supported in Vue 3, the Composition API is now the preferred approach for code reuse between components.

• Example:

  const mixin = {
    created() {
      console.log(1)
    }
  }
  
  createApp({
    created() {
      console.log(2)
    },
    mixins: [mixin]
  })
  
  // => 1
  // => 2
  

• See also: Mixins

extends

• Type: Object

• Details:

  Allows one component to extend another, inheriting its component options.

  From an implementation perspective, extends is almost identical to mixins. The component specified by extends will be treated as though it were the first mixin.

  However, extends and mixins express different intents. The mixins option is primarily used to compose chunks of functionality, whereas extends is primarily concerned with inheritance.

  As with mixins, any options will be merged using the relevant merge strategy.

• Example:

  const CompA = { ... }
  
  const CompB = {
    extends: CompA,
    ...
  }
  

provide / inject

• Type:

  • provide: Object | () => Object
  • inject: Array<string> | { [key: string]: string | Symbol | Object }

• Details:

  This pair of options are used together to allow an ancestor component to serve as a dependency injector for all its descendants, regardless of how deep the component hierarchy is, as long as they are in the same parent chain. If you are familiar with React, this is very similar to React's context feature.

  The provide option should be an object or a function that returns an object. This object contains the properties that are available for injection into its descendants. You can use ES2015 Symbols as keys in this object, but only in environments that natively support Symbol and Reflect.ownKeys.

  The inject option should be either:

  • an array of strings, or
  • an object where the keys are the local binding name and the value is either:
    • the key (string or Symbol) to search for in available injections, or
    • an object where:
      • the from property is the key (string or Symbol) to search for in available injections, and
      • the default property is used as fallback value

  Note: the provide and inject bindings are NOT reactive. This is intentional. However, if you pass down a reactive object, properties on that object do remain reactive.

• Example:

  // parent component providing 'foo'
  const Provider = {
    provide: {
      foo: 'bar'
    }
    // ...
  }
  
  // child component injecting 'foo'
  const Child = {
    inject: ['foo'],
    created() {
      console.log(this.foo) // => "bar"
    }
    // ...
  }
  

  With ES2015 Symbols, function provide and object inject:

  const s = Symbol()
  
  const Provider = {
    provide() {
      return {
        [s]: 'foo'
      }
    }
  }
  
  const Child = {
    inject: { s }
    // ...
  }
  

  Using an injected value as the default for a prop:

  const Child = {
    inject: ['foo'],
    props: {
      bar: {
        default() {
          return this.foo
        }
      }
    }
  }
  

  Using an injected value as data entry:

  const Child = {
    inject: ['foo'],
    data() {
      return {
        bar: this.foo
      }
    }
  }
  

  Injections can be optional with default value:

  const Child = {
    inject: {
      foo: { default: 'foo' }
    }
  }
  

  If it needs to be injected from a property with a different name, use from to denote the source property:

  const Child = {
    inject: {
      foo: {
        from: 'bar',
        default: 'foo'
      }
    }
  }
  

  Similar to prop defaults, you need to use a factory function for non-primitive values:

  const Child = {
    inject: {
      foo: {
        from: 'bar',
        default: () => [1, 2, 3]
      }
    }
  }
  

• See also: Provide / Inject

setup

• Type: Function

The setup function is a new component option. It serves as the entry point for using the Composition API inside components.

• Invocation Timing

  setup is called right after the initial props resolution when a component instance is created. Lifecycle-wise, it is called before the beforeCreate hook.

• Usage with Templates

  If setup returns an object, the properties on the object will be merged on to the render context for the component's template:

  <template>
    <div>{{ count }} {{ object.foo }}</div>
  </template>
  
  <script>
    import { ref, reactive } from 'vue'
  
    export default {
      setup() {
        const count = ref(0)
        const object = reactive({ foo: 'bar' })
  
        // expose to template
        return {
          count,
          object
        }
      }
    }
  </script>
  

  Note that refs returned from setup are automatically unwrapped when accessed in the template so there's no need for .value in templates.

• Usage with Render Functions / JSX

  setup can also return a render function, which can directly make use of reactive state declared in the same scope:

  import { h, ref, reactive } from 'vue'
  
  export default {
    setup() {
      const count = ref(0)
      const object = reactive({ foo: 'bar' })
  
      return () => h('div', [count.value, object.foo])
    }
  }
  

• Arguments

  The function receives the resolved props as its first argument:

  export default {
    props: {
      name: String
    },
    setup(props) {
      console.log(props.name)
    }
  }
  

  Note this props object is reactive - i.e. it is updated when new props are passed in, and can be observed and reacted upon using watchEffect or watch:

  export default {
    props: {
      name: String
    },
    setup(props) {
      watchEffect(() => {
        console.log(`name is: ` + props.name)
      })
    }
  }
  

  However, do NOT destructure the props object, as it will lose reactivity:

  export default {
    props: {
      name: String
    },
    setup({ name }) {
      watchEffect(() => {
        console.log(`name is: ` + name) // Will not be reactive!
      })
    }
  }
  

  The props object is immutable for userland code during development (will emit warning if user code attempts to mutate it).

  The second argument provides a context object which exposes various objects and functions that might be useful in setup:

  const MyComponent = {
    setup(props, context) {
      context.attrs
      context.slots
      context.emit
      context.expose
    }
  }
  

  attrs, slots, and emit are equivalent to the instance properties $attrs, $slots, and $emit respectively.

  attrs and slots are proxies to the corresponding values on the internal component instance. This ensures they always expose the latest values even after updates so that we can destructure them without worrying about accessing a stale reference:

  const MyComponent = {
    setup(props, { attrs }) {
      // a function that may get called at a later stage
      function onClick() {
        console.log(attrs.foo) // guaranteed to be the latest reference
      }
    }
  }
  

  expose, added in Vue 3.2, is a function that allows specific properties to be exposed via the public component instance. By default, the public instance retrieved using refs, $parent, or $root is equivalent to the internal instance used by the template. Calling expose will create a separate public instance with the properties specified:

  const MyComponent = {
    setup(props, { expose }) {
      const count = ref(0)
      const reset = () => count.value = 0
      const increment = () => count.value++
  
      // Only reset will be available externally, e.g. via $refs
      expose({
        reset
      })
  
      // Internally, the template has access to count and increment
      return { count, increment }
    }
  }
  

  There are a number of reasons for placing props as a separate first argument instead of including it in the context:

  • It's much more common for a component to use props than the other properties, and very often a component uses only props.

  • Having props as a separate argument makes it easier to type it individually without messing up the types of other properties on the context. It also makes it possible to keep a consistent signature across setup, render and plain functional components with TSX support.

• See also: Composition API