Emitting custom events in Vue

Introduction

Custom events let components talk to each other in Vue. They help a child component send messages to its parent.

When a child component needs to tell its parent that something happened, like a button click.

When you want to keep components separate but still let them share information.

When building reusable components that need to notify their users about changes.

When you want to handle user actions in a parent component but trigger them in a child.

When you want to keep your app organized by passing events up instead of changing data directly.

Syntax

Vue

<template>
  <button @click="$emit('eventName', eventData)">Click me</button>
</template>

<script setup>
// No extra code needed here
</script>

The $emit function sends an event from child to parent.

You can pass extra data as the second argument to $emit.

Examples

This emits an 'increment' event without extra data.

Vue

<template>
  <button @click="$emit('increment')">Add</button>
</template>

This emits an 'update' event and sends the number 5 as data.

Vue

<template>
  <button @click="$emit('update', 5)">Update to 5</button>
</template>

This emits 'input-changed' with the current input value whenever the user types.

Vue

<template>
  <input @input="$emit('input-changed', $event.target.value)" />
</template>

Sample Program

The child component emits a 'greet' event with a name. The parent listens and updates the message.

Vue

<template>
  <ChildComponent @greet="handleGreet" />
  <p>{{ message }}</p>
</template>

<script setup>
import { ref } from 'vue'
import ChildComponent from './ChildComponent.vue'

const message = ref('')

function handleGreet(name) {
  message.value = `Hello, ${name}!` 
}
</script>

<!-- ChildComponent.vue -->
<template>
  <button @click="$emit('greet', 'Vue Learner')">Say Hello</button>
</template>

<script setup>
// No extra code needed
</script>

Important Notes

Always listen for custom events in the parent using @eventName.

Use descriptive event names to keep your code clear.

Passing data with events helps parents know more about what happened.

Summary

Custom events let child components send messages to parents.

Use $emit('eventName', data) to send events.

Parents listen with @eventName="handler" to react.

Practice

1. What does the $emit method do in a Vue child component?

easy

A. It registers a new component globally.

B. It updates the child component's data directly.

C. It changes the route in a Vue Router setup.

D. It sends a custom event to the parent component.

Solution

1. Step 1: Understand the role of $emit

   The $emit method is used inside a child component to send a message or event to its parent.

2. Step 2: Identify the effect of $emit

   It triggers a custom event that the parent can listen to and respond accordingly.

3. Final Answer:

   It sends a custom event to the parent component. -> Option D

4. Quick Check:

   $emit sends events to parent [OK]

Hint: Remember: child uses $emit to talk to parent [OK]

Common Mistakes:

• Thinking $emit changes child data directly
• Confusing $emit with global component registration
• Assuming $emit changes routes

2. Which of the following is the correct syntax to emit a custom event named update with a payload 42 inside a Vue component?

easy

A. this.$emit('update', 42)

B. emit.$('update', 42)

C. $emit('update', 42)

D. this.emit('update', 42)

Solution

1. Step 1: Recall Vue event emission syntax

   In Vue, to emit an event from a component, you use this.$emit(eventName, payload).

2. Step 2: Match the correct syntax

   this.$emit('update', 42) correctly uses this.$emit('update', 42), which is the proper method call.

3. Final Answer:

   this.$emit('update', 42) -> Option A

4. Quick Check:

   Use this.$emit to emit events [OK]

Hint: Always use this.$emit to send events in Vue [OK]

Common Mistakes:

• Omitting 'this' before $emit
• Using incorrect method names like emit or emit.$
• Calling $emit without this context

3. Given this child component code:

export default {
  methods: {
    sendAlert() {
      this.$emit('alert', 'Hello!')
    }
  }
}

And the parent template:

<ChildComponent @alert="showMessage" />

What happens when sendAlert() is called?

medium

A. The parent method showMessage is called with 'Hello!' as argument.

B. Nothing happens because the event name is incorrect.

C. An error occurs because $emit cannot send strings.

D. The child component updates its own data with 'Hello!'.

Solution

1. Step 1: Understand event emission and listening

   The child emits an event named 'alert' with the string 'Hello!'. The parent listens for 'alert' and calls showMessage.

2. Step 2: Trace the event flow

   When sendAlert() runs, the parent receives the event and calls showMessage('Hello!').

3. Final Answer:

   The parent method showMessage is called with 'Hello!' as argument. -> Option A

4. Quick Check:

   Child emits event, parent listens and reacts [OK]

Hint: Child emits event, parent listens with @event [OK]

Common Mistakes:

• Thinking event names must be different in child and parent
• Believing $emit cannot send string data
• Assuming child updates itself on emit

4. Identify the error in this Vue child component method:

methods: {
  notify() {
    $emit('notify')
  }
}

medium

A. Event name 'notify' is reserved and cannot be used.

B. Missing this before $emit causes a ReferenceError.

C. The method should return a value after emitting.

D. The methods object cannot contain functions.

Solution

1. Step 1: Check method syntax for emitting events

   In Vue, $emit must be called with this context inside methods.

2. Step 2: Identify the missing this

   The code calls $emit('notify') without this, causing a ReferenceError.

3. Final Answer:

   Missing this before $emit causes a ReferenceError. -> Option B

4. Quick Check:

   Always use this.$emit inside methods [OK]

Hint: Use this.$emit, not just $emit inside methods [OK]

Common Mistakes:

• Omitting this before $emit
• Thinking event names are restricted
• Expecting methods to return values after emit

5. You want a child component to emit an event change only when a new value is different from the old one. Which code correctly implements this logic inside the child?

hard

A. methods: {
updateValue(newVal) {
if (newVal == this.value) {
this.$emit('change', newVal)
}
}
}

B. methods: {
updateValue(newVal) {
this.$emit('change', newVal)
}
}

C. methods: {
updateValue(newVal) {
if (newVal !== this.value) {
this.$emit('change', newVal)
}
}
}

D. methods: {
updateValue(newVal) {
this.emit('change', newVal)
}
}

Solution

1. Step 1: Understand the condition for emitting

   The event should emit only if the new value differs from the current this.value.

2. Step 2: Check each option's logic

   The code that uses if (newVal !== this.value) { this.$emit('change', newVal) } correctly emits only on change. Code that emits unconditionally sends every time. Code using if (newVal == this.value) emits when values match (wrong logic). Code with this.emit uses invalid syntax.

3. Final Answer:

   methods: { updateValue(newVal) { if (newVal !== this.value) { this.$emit('change', newVal) } } } -> Option C

4. Quick Check:

   Emit only if newVal differs from current [OK]

Hint: Emit event only when new value differs [OK]

Common Mistakes:

• Emitting event unconditionally
• Using == instead of !== for change detection
• Calling this.emit instead of this.$emit