Instance Methods

$watch

Arguments:
- {string | Function} source
- {Function | Object} callback
- {Object} options (optional)
  - {boolean} deep
  - {boolean} immediate
  - {string} flush
Returns: {Function} unwatch
Usage:

Watch a reactive property or a computed function on the component instance for changes. The callback gets called with the new value and the old value for the given property. We can only pass top-level data, props, or computed property name as a string. For more complex expressions or nested properties, use a function instead.

Example:

const app = createApp({
  data() {
    return {
      a: 1,
      b: 2,
      c: {
        d: 3,
        e: 4
      }
    }
  },
  created() {
    // top-level property name
    this.$watch('a', (newVal, oldVal) => {
      // do something
    })

    // function for watching a single nested property
    this.$watch(
      () => this.c.d,
      (newVal, oldVal) => {
        // do something
      }
    )

    // function for watching a complex expression
    this.$watch(
      // every time the expression `this.a + this.b` yields a different result,
      // the handler will be called. It's as if we were watching a computed
      // property without defining the computed property itself
      () => this.a + this.b,
      (newVal, oldVal) => {
        // do something
      }
    )
  }
})

When watched value is an object or array, any changes to its properties or elements won't trigger the watcher because they reference the same object/array:

const app = createApp({
  data() {
    return {
      article: {
        text: 'Vue is awesome!'
      },
      comments: ['Indeed!', 'I agree']
    }
  },
  created() {
    this.$watch('article', () => {
      console.log('Article changed!')
    })

    this.$watch('comments', () => {
      console.log('Comments changed!')
    })
  },
  methods: {
    // These methods won't trigger a watcher because we changed only a property of object/array,
    // not the object/array itself
    changeArticleText() {
      this.article.text = 'Vue 3 is awesome'
    },
    addComment() {
      this.comments.push('New comment')
    },

    // These methods will trigger a watcher because we replaced object/array completely
    changeWholeArticle() {
      this.article = { text: 'Vue 3 is awesome' }
    },
    clearComments() {
      this.comments = []
    }
  }
})

$watch returns an unwatch function that stops firing the callback:

const app = createApp({
  data() {
    return {
      a: 1
    }
  }
})

const vm = app.mount('#app')

const unwatch = vm.$watch('a', cb)
// later, teardown the watcher
unwatch()

Option: deep

To also detect nested value changes inside Objects, you need to pass in deep: true in the options argument. This option also can be used to watch array mutations.

Note: when mutating (rather than replacing) an Object or an Array and watch with deep option, the old value will be the same as new value because they reference the same Object/Array. Vue doesn't keep a copy of the pre-mutate value.
```
vm.$watch('someObject', callback, {
  deep: true
})
vm.someObject.nestedValue = 123
// callback is fired
```

Option: immediate

Passing in immediate: true in the option will trigger the callback immediately with the current value of the expression:

vm.$watch('a', callback, {
  immediate: true
})
// `callback` is fired immediately with current value of `a`

Note that with immediate option you won't be able to unwatch the given property on the first callback call.

// This will cause an error
const unwatch = vm.$watch(
  'value',
  function() {
    doSomething()
    unwatch()
  },
  { immediate: true }
)

If you still want to call an unwatch function inside the callback, you should check its availability first:

let unwatch = null

unwatch = vm.$watch(
  'value',
  function() {
    doSomething()
    if (unwatch) {
      unwatch()
    }
  },
  { immediate: true }
)

Option: flush

The flush option allows for greater control over the timing of the callback. It can be set to 'pre', 'post' or 'sync'.

The default value is 'pre', which specifies that the callback should be invoked before rendering. This allows the callback to update other values before the template runs.

The value 'post' can be used to defer the callback until after rendering. This should be used if the callback needs access to the updated DOM or child components via $refs.

If flush is set to 'sync', the callback will be called synchronously, as soon as the value changes.

For both 'pre' and 'post', the callback is buffered using a queue. The callback will only be added to the queue once, even if the watched value changes multiple times. The interim values will be skipped and won't be passed to the callback.

Buffering the callback not only improves performance but also helps to ensure data consistency. The watchers won't be triggered until the code performing the data updates has finished.

'sync' watchers should be used sparingly, as they don't have these benefits.

For more information about flush see Effect Flush Timing.
See also: Watchers

$emit

Arguments:
- {string} eventName
- ...args (optional)
Trigger an event on the current instance. Any additional arguments will be passed into the listener's callback function.

Examples:

Using $emit with only an event name:

<div id="emit-example-simple">
  <welcome-button v-on:welcome="sayHi"></welcome-button>
</div>

const app = createApp({
  methods: {
    sayHi() {
      console.log('Hi!')
    }
  }
})

app.component('welcome-button', {
  emits: ['welcome'],
  template: `
    <button v-on:click="$emit('welcome')">
      Click me to be welcomed
    </button>
  `
})

app.mount('#emit-example-simple')

Using $emit with additional arguments:

<div id="emit-example-argument">
  <advice-component v-on:advise="showAdvice"></advice-component>
</div>

const app = createApp({
  methods: {
    showAdvice(advice) {
      alert(advice)
    }
  }
})

app.component('advice-component', {
  emits: ['advise'],
  data() {
    return {
      adviceText: 'Some advice'
    }
  },
  template: `
    <div>
      <input type="text" v-model="adviceText">
      <button v-on:click="$emit('advise', adviceText)">
        Click me for sending advice
      </button>
    </div>
  `
})

app.mount('#emit-example-argument')

See also:
- emits option
- Emitting a Value With an Event

$forceUpdate

Usage:

Force the component instance to re-render. Note it does not affect all child components, only the instance itself and child components with inserted slot content.

$nextTick

Arguments:
- {Function} callback (optional)
Usage:

Defer the callback to be executed after the next DOM update cycle. Use it immediately after you've changed some data to wait for the DOM update. This is the same as the global nextTick, except that the callback's this context is automatically bound to the instance calling this method.

Example:

createApp({
  // ...
  methods: {
    // ...
    example() {
      // modify data
      this.message = 'changed'
      // DOM is not updated yet
      this.$nextTick(function() {
        // DOM is now updated
        // `this` is bound to the current instance
        this.doSomethingElse()
      })
    }
  }
})

See also: nextTick