Emitting custom events in Vue - Step-by-Step Execution

Concept Flow - Emitting custom events

Child Component Action

↓

Emit Custom Event

↓

Parent Listens to Event

↓

Parent Handles Event

↓

Update Parent State or Run Logic

The child component triggers an event, the parent listens and reacts, updating state or running code.

Execution Sample

Vue

/* Child.vue */
<script setup>
const emit = defineEmits(['notify'])
function sendNotification() {
  emit('notify', 'Hello from child')
}
</script>

<template>
  <button @click="sendNotification">Notify Parent</button>
</template>

Child component emits a 'notify' event with a message when button is clicked.

Execution Table

Step	Action	Event Emitted	Payload	Parent Reaction	Parent State Change
1	Child button clicked	notify	'Hello from child'	Parent method called	parentMessage updated
2	Parent updates message	none	none	UI shows new message	parentMessage = 'Hello from child'
3	No further events	none	none	No action	No change

💡 No more events emitted, parent state updated with child's message

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
parentMessage	''	''	'Hello from child'	'Hello from child'
eventEmitted	none	'notify'	none	none
payload	none	'Hello from child'	none	none

Key Moments - 2 Insights

Why does the parent update only after the child emits the event?

Because the parent listens for the custom event 'notify' and only runs its handler when that event is emitted, as shown in execution_table step 1.

What happens if the child emits an event with no listener in the parent?

The event is emitted but no parent reaction occurs, so no state changes happen. This is why the parent must explicitly listen for the event.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'parentMessage' after step 1?

A undefined

B 'Hello from child'

C '' (empty string)

D null

💡 Hint

Check variable_tracker column 'After Step 1' for 'parentMessage'

At which step does the parent update its state with the child's message?

A Step 1

B Step 2

C Step 3

D No update occurs

💡 Hint

See execution_table 'Parent State Change' column

If the child emitted an event named 'alert' instead of 'notify', what would happen given the current parent listener?

A Parent would ignore the event

B Parent would react normally

C Parent would throw an error

D Child would not emit the event

💡 Hint

Parent listens only to 'notify' event as per concept_flow and code sample

Concept Snapshot

Emitting custom events in Vue:
- Child uses defineEmits(['eventName']) to declare events
- Child calls emit('eventName', payload) to send event
- Parent listens with @eventName="handler"
- Parent handler runs when event fires, updating state or logic
- Enables child-to-parent communication cleanly

Full Transcript

In Vue, child components can send messages to their parents by emitting custom events. The child declares which events it can emit using defineEmits. When an action happens in the child, like a button click, it calls emit with the event name and optional data. The parent listens for this event using the @eventName syntax in the template. When the event fires, the parent's handler runs, often updating its state or running some logic. This flow allows the child to notify the parent without directly changing the parent's data, keeping components decoupled and easy to manage.