Options: State #

data #

A function that returns the initial reactive state for the component instance.

• Type

  ts

  interface ComponentOptions {
    data?(
      this: ComponentPublicInstance,
      vm: ComponentPublicInstance
    ): object
  }
  

• Details

  The function is expected to return a plain JavaScript object, which will be made reactive by Vue. After the instance is created, the reactive data object can be accessed as this.$data. The component instance also proxies all the properties found on the data object, so this.a will be equivalent to this.$data.a.

  All top-level data properties must be included in the returned data object. Adding new properties to this.$data is possible, but it is not recommended. If the desired value of a property is not yet available then an empty value such as undefined or null should be included as a placeholder to ensure that Vue knows that the property exists.

  Properties that start with _ or $ will not be proxied on the component instance because they may conflict with Vue's internal properties and API methods. You will have to access them as this.$data._property.

  It is not recommended to return objects with their own stateful behavior like browser API objects and prototype properties. The returned object should ideally be a plain object that only represents the state of the component.

• Example

  js

  export default {
    data() {
      return { a: 1 }
    },
    created() {
      console.log(this.a) // 1
      console.log(this.$data) // { a: 1 }
    }
  }
  

  Note that if you use an arrow function with the data property, this won't be the component's instance, but you can still access the instance as the function's first argument:

  js

  data: (vm) => ({ a: vm.myProp })
  

• See also: Reactivity in Depth

props #

Declare the props of a component.

• Type

  ts

  interface ComponentOptions {
    props?: ArrayPropsOptions | ObjectPropsOptions
  }
  
  type ArrayPropsOptions = string[]
  
  type ObjectPropsOptions = { [key: string]: Prop }
  
  type Prop<T = any> = PropOptions<T> | PropType<T> | null
  
  interface PropOptions<T> {
    type?: PropType<T>
    required?: boolean
    default?: T | ((rawProps: object) => T)
    validator?: (value: unknown) => boolean
  }
  
  type PropType<T> = { new (): T } | { new (): T }[]
  

  Types are simplified for readability.

• Details

  In Vue, all component props need to be explicitly declared. Component props can be declared in two forms:

  • Simple form using an array of strings
  • Full form using an object where each property key is the name of the prop, and the value is the prop's type (a constructor function) or advanced options.

  With object-based syntax, each prop can further define the following options:

  • type: Can be one of the following native constructors: String, Number, Boolean, Array, Object, Date, Function, Symbol, any custom constructor function or an array of those. In development mode, Vue will check if a prop's value matches the declared type, and will throw a warning if it doesn't. Voir la validation de prop pour plus de détails.

    Also note that a prop with Boolean type affects its value casting behavior in both development and production. Voir la conversion en booléen pour plus de détails.

  • default: Specifies a default value for the prop when it is not passed by the parent or has undefined value. Object or array defaults must be returned using a factory function. The factory function also receives the raw props object as the argument.

  • required: Defines if the prop is required. In a non-production environment, a console warning will be thrown if this value is truthy and the prop is not passed.

  • validator: Custom validator function that takes the prop value as the sole argument. In development mode, a console warning will be thrown if this function returns a falsy value (i.e. the validation fails).

• Example

  Simple declaration:

  js

  export default {
    props: ['size', 'myMessage']
  }
  

  Object declaration with validations:

  js

  export default {
    props: {
      // type check
      height: Number,
      // type check plus other validations
      age: {
        type: Number,
        default: 0,
        required: true,
        validator: (value) => {
          return value >= 0
        }
      }
    }
  }
  

• See also:

  • Guide - Props
  • Guide - Typing Component Props

computed #

Declare computed properties to be exposed on the component instance.

• Type

  ts

  interface ComponentOptions {
    computed?: {
      [key: string]: ComputedGetter<any> | WritableComputedOptions<any>
    }
  }
  
  type ComputedGetter<T> = (
    this: ComponentPublicInstance,
    vm: ComponentPublicInstance
  ) => T
  
  type ComputedSetter<T> = (
    this: ComponentPublicInstance,
    value: T
  ) => void
  
  type WritableComputedOptions<T> = {
    get: ComputedGetter<T>
    set: ComputedSetter<T>
  }
  

• Details

  The option accepts an object where the key is the name of the computed property, and the value is either a computed getter, or an object with get and set methods (for writable computed properties).

  All getters and setters have their this context automatically bound to the component instance.

  Note that if you use an arrow function with a computed property, this won't point to the component's instance, but you can still access the instance as the function's first argument:

  js

  export default {
    computed: {
      aDouble: (vm) => vm.a * 2
    }
  }
  

• Example

  js

  export default {
    data() {
      return { a: 1 }
    },
    computed: {
      // readonly
      aDouble() {
        return this.a * 2
      },
      // writable
      aPlus: {
        get() {
          return this.a + 1
        },
        set(v) {
          this.a = v - 1
        }
      }
    },
    created() {
      console.log(this.aDouble) // => 2
      console.log(this.aPlus) // => 2
  
      this.aPlus = 3
      console.log(this.a) // => 2
      console.log(this.aDouble) // => 4
    }
  }
  

• See also:

  • Guide - Propriétés calculées
  • Guide - Typing Computed Properties

methods #

Declare methods to be mixed into the component instance.

• Type

  ts

  interface ComponentOptions {
    methods?: {
      [key: string]: (this: ComponentPublicInstance, ...args: any[]) => any
    }
  }
  

• Details

  Declared methods can be directly accessed on the component instance, or used in template expressions. All methods have their this context automatically bound to the component instance, even when passed around.

  Avoid using arrow functions when declaring methods, as they will not have access to the component instance via this.

• Example

  js

  export default {
    data() {
      return { a: 1 }
    },
    methods: {
      plus() {
        this.a++
      }
    },
    created() {
      this.plus()
      console.log(this.a) // => 2
    }
  }
  

• See also: Gestion d'événement

watch #

Declare watch callbacks to be invoked on data change.

• Type

  ts

  interface ComponentOptions {
    watch?: {
      [key: string]: WatchOptionItem | WatchOptionItem[]
    }
  }
  
  type WatchOptionItem = string | WatchCallback | ObjectWatchOptionItem
  
  type WatchCallback<T> = (
    value: T,
    oldValue: T,
    onCleanup: (cleanupFn: () => void) => void
  ) => void
  
  type ObjectWatchOptionItem = {
    handler: WatchCallback | string
    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

  The watch option expects an object where keys are the reactive component instance properties to watch (e.g. properties declared via data or computed) — and values are the corresponding callbacks. The callback receives the new value and the old value of the watched source.

  In addition to a root-level property, the key can also be a simple dot-delimited path, e.g. a.b.c. Note that this usage does not support complex expressions - only dot-delimited paths are supported. If you need to watch complex data sources, use the imperative $watch() API instead.

  The value can also be a string of a method name (declared via methods), or an object that contains additional options. When using the object syntax, the callback should be declared under the handler field. Additional options include:

  • 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 or an array, so that the callback fires on deep mutations. See les observateurs profonds.
  • flush: adjust the callback's flush timing. See Callback Flush Timing and watchEffect().
  • onTrack / onTrigger: debug the watcher's dependencies. See Watcher Debugging.

  Avoid using arrow functions when declaring watch callbacks as they will not have access to the component instance via this.

• Example

  js

  export default {
    data() {
      return {
        a: 1,
        b: 2,
        c: {
          d: 4
        },
        e: 5,
        f: 6
      }
    },
    watch: {
      // watching top-level property
      a(val, oldVal) {
        console.log(`new: ${val}, old: ${oldVal}`)
      },
      // string method name
      b: 'someMethod',
      // the callback will be called whenever any of the watched object properties change regardless of their nested depth
      c: {
        handler(val, oldVal) {
          console.log('c changed')
        },
        deep: true
      },
      // watching a single nested property:
      'c.d': function (val, oldVal) {
        // do something
      },
      // the callback will be called immediately after the start of the observation
      e: {
        handler(val, oldVal) {
          console.log('e changed')
        },
        immediate: true
      },
      // you can pass array of callbacks, they will be called one-by-one
      f: [
        'handle1',
        function handle2(val, oldVal) {
          console.log('handle2 triggered')
        },
        {
          handler: function handle3(val, oldVal) {
            console.log('handle3 triggered')
          }
          /* ... */
        }
      ]
    },
    methods: {
      someMethod() {
        console.log('b changed')
      },
      handle1() {
        console.log('handle 1 triggered')
      }
    },
    created() {
      this.a = 3 // => new: 3, old: 1
    }
  }
  

• See also: Observateurs

emits #

Declare the custom events emitted by the component.

• Type

  ts

  interface ComponentOptions {
    emits?: ArrayEmitsOptions | ObjectEmitsOptions
  }
  
  type ArrayEmitsOptions = string[]
  
  type ObjectEmitsOptions = { [key: string]: EmitValidator | null }
  
  type EmitValidator = (...args: unknown[]) => boolean
  

• Details

  Emitted events can be declared in two forms:

  • Simple form using an array of strings
  • Full form using an object where each property key is the name of the event, and the value is either null or a validator function.

  The validation function will receive the additional arguments passed to the component's $emit call. For example, if this.$emit('foo', 1) is called, the corresponding validator for foo will receive the argument 1. The validator function should return a boolean to indicate whether the event arguments are valid.

  Note that the emits option affects which event listeners are considered component event listeners, rather than native DOM event listeners. The listeners for declared events will be removed from the component's $attrs object, so they will not be passed through to the component's root element. See Attributs implicitement déclarés for more details.

• Example

  Array syntax:

  js

  export default {
    emits: ['check'],
    created() {
      this.$emit('check')
    }
  }
  

  Object syntax:

  js

  export default {
    emits: {
      // no validation
      click: null,
  
      // with validation
      submit: (payload) => {
        if (payload.email && payload.password) {
          return true
        } else {
          console.warn(`Invalid submit event payload!`)
          return false
        }
      }
    }
  }
  

• See also:

  • Guide - Attributs implicitement déclarés
  • Guide - Typing Component Emits

expose #

Declare exposed public properties when the component instance is accessed by a parent via template refs.

• Type

  ts

  interface ComponentOptions {
    expose?: string[]
  }
  

• Details

  By default, a component instance exposes all instance properties to the parent when accessed via $parent, $root, or template refs. This can be undesirable, since a component most likely has internal state or methods that should be kept private to avoid tight coupling.

  The expose option expects a list of property name strings. When expose is used, only the properties explicitly listed will be exposed on the component's public instance.

  expose only affects user-defined properties - it does not filter out built-in component instance properties.

• Example

  js

  export default {
    // only `publicMethod` will be available on the public instance
    expose: ['publicMethod'],
    methods: {
      publicMethod() {
        // ...
      },
      privateMethod() {
        // ...
      }
    }
  }