Options: State

data

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

Type

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

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:

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

See also: Reactivity in Depth

props

Declare the props of a component.

Type

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. See Prop Validation for more details.
  Also note that a prop with Boolean type affects its value casting behavior in both development and production. See Boolean Casting for more details.
- 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:

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

Object declaration with validations:

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: Props

computed

Declare computed properties to be exposed on the component instance.

Type

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

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: Computed Properties

methods

Declare methods to be mixed into the component instance.

Type

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

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

See also: Event Handling

watch

Declare watch callbacks to be invoked on data change.

Type

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

Example

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: Watchers

emits

Declare the custom events emitted by the component.

Type

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 Fallthrough Attributes for more details.

Example

Array syntax:

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

Object syntax:

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: Fallthrough Attributes

expose

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

Type

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

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

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Options: State #

data #

props #

computed #

methods #

watch #

emits #

expose #

Options: State

data

props

computed

methods

watch

emits

expose