Emitting custom events in Vue - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to emit a custom event named 'update'.

Vue

<script setup>
const emit = defineEmits(['update'])

function sendUpdate() {
  emit([1])
}
</script>

Drag options to blanks, or click blank then click option'

A'update'

B'click'

C'submit'

D'change'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the event name.

Using an event name not declared in defineEmits.

The emit function is called with the event name to emit. Here, the event name is 'update'.

2fill in blank

medium

Complete the code to declare an event named 'submitForm' using defineEmits.

Vue

<script setup>
const emit = defineEmits([[1]])
</script>

Drag options to blanks, or click blank then click option'

A'clickEvent'

B'submitForm'

C'inputChange'

D'loadData'

Attempts:

3 left

💡 Hint

Common Mistakes

Using event names not matching the emitted event.

Omitting quotes around the event name.

The defineEmits function takes an array of event names. Here, we declare the event 'submitForm'.

3fill in blank

hard

Fix the error in emitting the event 'closeModal' with a payload.

Vue

<script setup>
const emit = defineEmits(['closeModal'])

function close() {
  emit([1], true)
}
</script>

Drag options to blanks, or click blank then click option'

A'closeModal'

BcloseModal

C'closemodal'

D'close_modal'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the event name without quotes.

Using incorrect casing in the event name.

The event name must be a string matching the declared event. It should be 'closeModal' with exact casing and quotes.

4fill in blank

hard

Fill both blanks to emit 'saveData' event with a payload object containing 'id' and 'value'.

Vue

<script setup>
const emit = defineEmits([[1]])

function save() {
  emit([2], { id: 1, value: 'test' })
}
</script>

Drag options to blanks, or click blank then click option'

A'saveData'

B'updateData'

C'deleteData'

D'loadData'

Attempts:

3 left

💡 Hint

Common Mistakes

Using different event names in declaration and emission.

Omitting quotes around event names.

Both blanks require the event name 'saveData' to declare and emit the event correctly.

5fill in blank

hard

Fill all three blanks to declare events 'open', 'close', and 'toggle' and emit 'toggle' with a boolean payload.

Vue

<script setup>
const emit = defineEmits([[1], [2], [3]])

function toggleState() {
  emit('toggle', true)
}
</script>

Drag options to blanks, or click blank then click option'

A'open'

B'close'

C'toggle'

D'submit'

Attempts:

3 left

💡 Hint

Common Mistakes

Missing one or more event names in the declaration.

Using event names not matching the emitted event.

The events 'open', 'close', and 'toggle' are declared in defineEmits. The function emits the 'toggle' event with a payload.

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