vuejs update parent data from child component

Question

I'm starting to play with vuejs (2.0). I built a simple page with one component in it. The page has one Vue instance with data. On that page I registered and added the component to html. The component has one input[type=text]. I want that value to reflect on the parent (main Vue instance).

How do I correctly update the component's parent data? Passing a bound prop from the parent is not good and throws some warnings to the console. They have something in their doc but it is not working.

Answer 1

From the documentation:

In Vue.js, the parent-child component relationship can be summarized as props down, events up. The parent passes data down to the child via props, and the child sends messages to the parent via events. Let’s see how they work next.

How to pass props

Following is the code to pass props to a child element:

<div>
  <input v-model="parentMsg">
  <br>
  <child v-bind:my-message="parentMsg"></child>
</div>

How to emit event

HTML:

<div id="counter-event-example">
  <p>{{ total }}</p>
  <button-counter v-on:increment="incrementTotal"></button-counter>
  <button-counter v-on:increment="incrementTotal"></button-counter>
</div>

JS:

Vue.component('button-counter', {
  template: '<button v-on:click="increment">{{ counter }}</button>',
  data: function () {
    return {
      counter: 0
    }
  },
  methods: {
    increment: function () {
      this.counter += 1
      this.$emit('increment')
    }
  },
})
new Vue({
  el: '#counter-event-example',
  data: {
    total: 0
  },
  methods: {
    incrementTotal: function () {
      this.total += 1
    }
  }
})

Answer 2

It is also possible to pass props as Object or Array. In this case data will be two-way binded:

(This is noted at the end of topic: https://vuejs.org/v2/guide/components.html#One-Way-Data-Flow )

Vue.component('child', {
  template: '#child',
  props: {post: Object},
  methods: {
    updateValue: function () {
      this.$emit('changed');
    }
  }
});

new Vue({
  el: '#app',
  data: {
    post: {msg: 'hello'},
    changed: false
  },
  methods: {
    saveChanges() {
        this.changed = true;
    }
  }
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.13/vue.js"></script>

<div id="app">
  <p>Parent value: {{post.msg}}</p>
  <p v-if="changed == true">Parent msg: Data been changed - received signal from child!</p>
  <child :post="post" v-on:changed="saveChanges"></child>
</div>

<template id="child">
   <input type="text" v-model="post.msg" v-on:input="updateValue()">
</template>

Answer 3

In child component: this.$emit('eventname', this.variable)

In parent component:

<component @eventname="updateparent"></component>

methods: {
    updateparent(variable) {
        this.parentvariable = variable
    }
}

Answer 4

I agree with the event emitting and v-model answers for those above. However, I thought I would post what I found about components with multiple form elements that want to emit back to their parent since this seems one of the first articles returned by google.

I know the question specifies a single input, but this seemed the closest match and might save people some time with similar vue components. Also, no one has mentioned the .sync modifier yet.

As far as I know, the v-model solution is only suited to one input returning to their parent. I took a bit of time looking for it but Vue (2.3.0) documentation does show how to sync multiple props sent into the component back to the parent (via emit of course).

It is appropriately called the .sync modifier.

Here is what the documentation says:

In some cases, we may need “two-way binding” for a prop. Unfortunately, true two-way binding can create maintenance issues, because child components can mutate the parent without the source of that mutation being obvious in both the parent and the child.

That’s why instead, we recommend emitting events in the pattern of update:myPropName. For example, in a hypothetical component with a title prop, we could communicate the intent of assigning a new value with:

this.$emit('update:title', newTitle)

Then the parent can listen to that event and update a local data property, if it wants to. For example:

<text-document   
 v-bind:title="doc.title"  
 v-on:update:title="doc.title = $event"
></text-document>

For convenience, we offer a shorthand for this pattern with the .sync modifier:

<text-document v-bind:title.sync="doc.title"></text-document>

You can also sync multiple at a time by sending through an object. Check out the documentation here

Answer 5

Two-way binding has been deprecated in Vue 2.0 in favor of using a more event-driven architecture. In general, a child should not mutate its props. Rather, it should $emit events and let the parent respond to those events.

In your specific case, you could use a custom component with v-model. This is a special syntax which allows for something close to two-way binding, but is actually a shorthand for the event-driven architecture described above. You can read about it here -> https://vuejs.org/v2/guide/components.html#Form-Input-Components-using-Custom-Events.

Here's a simple example:

Vue.component('child', {
  template: '#child',
  
  //The child has a prop named 'value'. v-model will automatically bind to this prop
  props: ['value'],
  methods: {
    updateValue: function (value) {
      this.$emit('input', value);
    }
  }
});

new Vue({
  el: '#app',
  data: {
    parentValue: 'hello'
  }
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.13/vue.js"></script>

<div id="app">
  <p>Parent value: {{parentValue}}</p>
  <child v-model="parentValue"></child>
</div>

<template id="child">
   <input type="text" v-bind:value="value" v-on:input="updateValue($event.target.value)">
</template>

---

The docs state that

<custom-input v-bind:value="something" v-on:input="something = arguments[0]"></custom-input>

is equivalent to

<custom-input v-model="something"></custom-input>

That is why the prop on the child needs to be named value, and why the child needs to $emit an event named input.