Typing emits in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the emitted event payload type in this Vue component?

Consider this Vue 3 component using defineEmits with TypeScript. What is the type of the payload emitted by the update event?

Vue

const emit = defineEmits<{ (e: 'update', value: number): void }>()

function updateValue() {
  emit('update', 42)
}

A A number value

B An object with a number property

C A string value

D No payload

💡 Hint

Look at the second parameter type in the emit definition.

The emit is typed to accept an event named 'update' with a payload of type number. So the emitted payload is a number.

📝 Syntax

intermediate

Which option correctly types multiple emits in Vue 3?

You want to type two emits: 'save' with no payload and 'delete' with a string payload. Which defineEmits syntax is correct?

A const emit = defineEmits({ save: null, delete: String })

B const emit = defineEmits(['save', 'delete'])

C const emit = defineEmits<{ save: void; delete: string }>()

D const emit = defineEmits<{ (e: 'save'): void; (e: 'delete', id: string): void }>()

💡 Hint

Use function overload syntax for multiple typed emits.

Option D uses function overloads to type each event and its payload correctly. Other options are either untyped or invalid.

🔧 Debug

advanced

Why does this Vue component emit cause a TypeScript error?

This component emits an event 'change' with a string payload, but TypeScript shows an error on emit call. Why?

Vue

const emit = defineEmits<{ (e: 'change', value: number): void }>()

emit('change', 'hello')

A Emits must not have any payload

B The emitted payload type does not match the defined type

C The emit function is not called inside setup

D The event name 'change' is not defined in emits

💡 Hint

Check the type of the second argument in the emit call.

The emit is defined to accept a number payload for 'change', but the code emits a string, causing a type error.

❓ state_output

advanced

What is the output when emitting an event with typed emits in Vue 3?

Given this component, what will be logged when trigger is called?

Vue

const emit = defineEmits<{ (e: 'log', message: string): void }>()

function trigger() {
  emit('log', 'Hello Vue')
  console.log('Event emitted')
}

trigger()

A TypeScript error

B No output

C Event emitted

D Hello Vue

💡 Hint

Emitting an event does not print anything by itself.

The emit call triggers an event but does not produce console output. Only the console.log prints 'Event emitted'.

🧠 Conceptual

expert

Why use defineEmits with TypeScript in Vue 3?

Which is the main benefit of typing emits with defineEmits in Vue 3?

A It enforces correct event names and payload types at compile time

B It automatically registers events globally

C It replaces the need for props in components

D It enables emitting events without any parameters

💡 Hint

Think about what TypeScript helps prevent.

Typing emits helps catch mistakes like wrong event names or wrong payload types before running the app.

Practice

1. What is the main purpose of using defineEmits in a Vue component?

easy

A. To declare which events the component can emit and their payload types

B. To define the component's reactive state variables

C. To register global Vue plugins

D. To import external libraries into the component

Solution

1. Step 1: Understand the role of defineEmits

   defineEmits is used inside <script setup> to specify which events a component can send out.

2. Step 2: Identify what it declares

   It declares event names and their payload types to improve type safety and tooling support.

3. Final Answer:

   To declare which events the component can emit and their payload types -> Option A

4. Quick Check:

   Typing emits = declare events and payloads [OK]

Hint: defineEmits declares events your component can send [OK]

Common Mistakes:

• Confusing defineEmits with state or props
• Thinking it registers plugins
• Using it to import libraries

2. Which of the following is the correct syntax to type an emit event named submit with a payload of type string in Vue 3 <script setup>?

easy

A. const emit = defineEmits(['submit: string'])

B. const emit = defineEmits({ submit: string })

C. const emit = defineEmits('submit', string)

D. const emit = defineEmits<{ submit: (payload: string) => void }>()

Solution

1. Step 1: Recall the correct typing syntax for emits

   Vue 3 uses defineEmits with a TypeScript type object describing event names and their payload function signatures.

2. Step 2: Match the correct syntax

   The correct syntax is defineEmits<{ submit: (payload: string) => void }>() which types the submit event with a string payload.

3. Final Answer:

   const emit = defineEmits<{ submit: (payload: string) => void }>() -> Option D

4. Quick Check:

   Typed emits use object with event function signatures [OK]

Hint: Use defineEmits with typed event function signatures [OK]

Common Mistakes:

• Passing event and type as separate arguments
• Using array with type inside string
• Using object with type directly instead of function signature

3. Given this Vue <script setup> code:

const emit = defineEmits<{ update: (value: number) => void }>();

emit('update', 42);

What will happen if you try to emit update with a string payload like emit('update', 'hello')?

medium

A. Vue will throw a runtime error when emitting

B. TypeScript will show an error because the payload type is incorrect

C. The event will emit successfully with the string payload

D. The event will be ignored silently

Solution

1. Step 1: Understand the typed emit signature

   The update event expects a payload of type number as declared in defineEmits<{ update: (value: number) => void }>().

2. Step 2: Check what happens if payload type mismatches

   Passing a string like 'hello' violates the type signature, so TypeScript will report a compile-time error before running.

3. Final Answer:

   TypeScript will show an error because the payload type is incorrect -> Option B

4. Quick Check:

   Typed emits enforce payload types at compile time [OK]

Hint: Typed emits catch wrong payload types before runtime [OK]

Common Mistakes:

• Thinking Vue throws runtime error instead of TS error
• Assuming event emits anyway without error
• Confusing payload type with event name

4. Identify the error in this Vue <script setup> code snippet:

const emit = defineEmits<{ save: (data: object) => void }>();

emit('save');

medium

A. Missing payload argument when emitting 'save' event

B. Incorrect event name 'save' should be 'Save'

C. defineEmits should not use type annotations

D. emit function is not defined

Solution

1. Step 1: Check the typed emit signature

   The save event expects a payload argument of type object.

2. Step 2: Analyze the emit call

   The call emit('save') is missing the required payload argument, causing a type error.

3. Final Answer:

   Missing payload argument when emitting 'save' event -> Option A

4. Quick Check:

   Typed emits require payloads as declared [OK]

Hint: Emit calls must include all typed payload arguments [OK]

Common Mistakes:

• Omitting required payload argument
• Using wrong event name casing
• Thinking type annotations are invalid here

5. You want to type a Vue component emit that can send either a string or number payload with the event name change. Which is the correct way to type this using defineEmits?

hard

A. const emit = defineEmits<{ change: (value: string & number) => void }>()

B. const emit = defineEmits<{ change: (value: string) => void; change: (value: number) => void }>()

C. const emit = defineEmits<{ change: (value: string | number) => void }>()

D. const emit = defineEmits<{ change: (value: any) => void }>()

Solution

1. Step 1: Understand union types for payloads

   To allow multiple types for a payload, use a union type with |, e.g., string | number.

2. Step 2: Check the options for correct union usage

   const emit = defineEmits<{ change: (value: string | number) => void }>() correctly types the change event with a payload that can be either string or number.

3. Step 3: Identify incorrect options

   const emit = defineEmits<{ change: (value: string) => void; change: (value: number) => void }>() repeats the same event name twice, which is invalid. const emit = defineEmits<{ change: (value: string & number) => void }>() uses intersection & which means both types simultaneously (impossible). const emit = defineEmits<{ change: (value: any) => void }>() uses any which loses type safety.

4. Final Answer:

   const emit = defineEmits<{ change: (value: string | number) => void }>() -> Option C

5. Quick Check:

   Use union types for multiple payload types [OK]

Hint: Use | to combine payload types in defineEmits [OK]

Common Mistakes:

• Repeating event names in type object
• Using & instead of | for union types
• Using any which disables type safety