Script setup syntax in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the <script setup> syntax in Vue 3?

It is a simpler and more concise way to write the component's script logic using the Composition API without the need to explicitly return values or define a setup() function.

beginner

How do you declare reactive state variables inside <script setup>?

You use the ref() or reactive() functions imported from Vue to create reactive state variables directly inside the <script setup> block.

intermediate

Can you use props directly inside <script setup> without defining them explicitly?

No, you must define props using the defineProps() helper function to declare the expected props and their types inside <script setup>.

intermediate

How do you emit events from a component using <script setup>?

You use the defineEmits() helper function to declare the events your component can emit, then call the returned emit function to send events.

beginner

What is one advantage of using <script setup> over the traditional setup() function?

It reduces boilerplate code by removing the need to return variables and functions explicitly, making the component code cleaner and easier to read.

Which function is used to declare props inside <script setup>?

A useProps()

B props()

C defineProps()

D declareProps()

The defineProps() function is the official way to declare props in <script setup>.

How do you create a reactive variable inside <script setup>?

A Using reactiveState()

B Using reactiveVar()

C Using useState()

D Using ref() or reactive()

Vue provides ref() and reactive() to create reactive variables.

What must you do to emit an event from a component using <script setup>?

A Events cannot be emitted in <code>&lt;script setup&gt;</code>

B Use defineEmits() and call the emit function

C Use emitEvent() helper

D Call emit() directly without declaration

You declare events with defineEmits() and then call the returned emit function.

Which of these is NOT a benefit of <script setup>?

A Supports class-based components

B Automatic return of variables

C Less boilerplate code

D Simpler syntax for Composition API

Vue 3 Composition API and <script setup> do not support class-based components.

Where do you write the <script setup> block in a Vue component?

A Inside the <code>&lt;script&gt;</code> tag with the setup attribute

B Inside the <code>&lt;template&gt;</code> tag

C Inside the <code>&lt;style&gt;</code> tag

D Inside a separate JavaScript file only

You write <script setup> as a <script> tag with the setup attribute.

Explain how to define props and emit events using <script setup> in Vue 3.

Think about how you tell Vue what props and events your component uses.

Describe the main advantages of using <script setup> compared to the traditional setup function.

Focus on how it makes writing components easier and cleaner.

Practice

1. What is the main advantage of using <script setup> in a Vue component?

easy

A. It requires class-based components instead of functions.

B. It automatically adds CSS styles to the component.

C. It disables the use of reactive variables.

D. It removes the need for export default and simplifies code.

Solution

1. Step 1: Understand the purpose of <script setup>

   This syntax is designed to simplify Vue components by removing boilerplate code like export default.

2. Step 2: Compare options with this purpose

   Only It removes the need for export default and simplifies code. correctly states that it removes the need for export default and simplifies code.

3. Final Answer:

   It removes the need for export default and simplifies code. -> Option D

4. Quick Check:

   Script setup simplifies code = A [OK]

Hint: Remember script setup removes export default for simpler code [OK]

Common Mistakes:

• Thinking script setup adds styles automatically
• Believing reactive variables are disabled
• Confusing script setup with class-based components

2. Which of the following is the correct way to declare a reactive variable inside <script setup>?

easy

A. let count = reactive(0);

B. const count = ref(0);

C. var count = reactiveRef(0);

D. const count = reactiveVar(0);

Solution

1. Step 1: Recall Vue 3 Composition API reactive variable syntax

   Reactive variables use ref() to wrap primitive values like numbers.

2. Step 2: Check each option

   const count = ref(0); uses ref(0), which is correct. Options A, C, and D use incorrect or non-existent functions.

3. Final Answer:

   const count = ref(0); -> Option B

4. Quick Check:

   Use ref() for reactive primitives = B [OK]

Hint: Use ref() for reactive primitives inside script setup [OK]

Common Mistakes:

• Using reactive() for primitive values
• Using undefined functions like reactiveRef or reactiveVar
• Using var instead of const or let

3. Given the following <script setup> code, what will be rendered inside the template?

<script setup>
import { ref } from 'vue'
const message = ref('Hello')
message.value = 'Hi'
</script>

<template>
  <p>{{ message }}</p>
</template>

medium

A. <p>Hello</p>

B. Error: message is not defined

C. <p>Hi</p>

D. <p>[object Object]</p>

Solution

1. Step 1: Understand how ref works in template interpolation

   In Vue, a ref is an object with a value property. To display the inner value, you must use message.value in script or just message in template because Vue unwraps refs in templates.

2. Step 2: Check the template interpolation

   The template uses {{ message }}. Vue unwraps refs here, so it should display the string inside message.value, which is 'Hi'.

3. Step 3: Identify the actual behavior

   In Vue 3 with <script setup>, refs are unwrapped automatically in templates, so it should display 'Hi'. The code sets message.value = 'Hi' correctly, so the output is 'Hi'.

4. Final Answer:

   <p>Hi</p> -> Option C

5. Quick Check:

   Template unwraps ref = Hi [OK]

Hint: Remember Vue unwraps refs automatically in templates [OK]

Common Mistakes:

• Thinking you must write message.value in template
• Confusing ref object with its value
• Assuming message is undefined

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

<script setup>
import { reactive } from 'vue'
const state = reactive({ count: 0 })
function increment() {
  state.count++
}
</script>

<template>
  <button @click="increment()">Increment</button>
  <p>Count: {{ state.count }}</p>
</template>

medium

A. There is no error; the code works correctly.

B. The increment function is not exposed to the template and cannot be called.

C. The reactive object should be declared with ref() instead of reactive().

D. The @click event syntax is incorrect; it should be v-on:click.

Solution

1. Step 1: Analyze the reactive state and function

   The code uses reactive to create a reactive object state with a count property. The increment function modifies state.count.

2. Step 2: Check template usage and event binding

   The template uses @click="increment()" which is valid shorthand for v-on:click. The function increment is defined in <script setup> and is automatically exposed to the template.

3. Final Answer:

   There is no error; the code works correctly. -> Option A

4. Quick Check:

   Script setup exposes functions automatically = C [OK]

Hint: Functions in script setup are auto-exposed to template [OK]

Common Mistakes:

• Thinking reactive must be replaced by ref for objects
• Believing functions need explicit export
• Confusing @click with invalid syntax

5. You want to create a Vue component using <script setup> that fetches data asynchronously when the component mounts and stores it in a reactive variable. Which is the correct way to do this?

hard

A. <script setup>
import { ref, onMounted } from 'vue'
const data = ref(null)
onMounted(async () => {
const res = await fetch('https://api.example.com/data')
data.value = await res.json()
})
</script>

B. <script setup>
import { reactive } from 'vue'
const data = reactive(null)
async function fetchData() {
const res = await fetch('https://api.example.com/data')
data = await res.json()
}
fetchData()
</script>

C. <script setup>
import { ref } from 'vue'
const data = ref(null)
fetch('https://api.example.com/data').then(res => {
data = res.json()
})
</script>

D. <script setup>
import { ref, onMounted } from 'vue'
const data = ref(null)
onMounted(() => {
fetch('https://api.example.com/data').then(res => res.json()).then(json => {
data.value = json
})
})
</script>

Solution

1. Step 1: Understand how to fetch data on component mount

   Use the onMounted lifecycle hook to run code after the component is ready.

2. Step 2: Check reactive variable usage and async fetch

   Use ref(null) for reactive data. Inside onMounted, use async/await to fetch and assign data to data.value.

3. Step 3: Evaluate each option

   <script setup>
   import { ref, onMounted } from 'vue'
   const data = ref(null)
   onMounted(async () => {
     const res = await fetch('https://api.example.com/data')
     data.value = await res.json()
   })
   </script>

   correctly uses ref, onMounted, and async/await with proper assignment to data.value.

   <script setup>
   import { reactive } from 'vue'
   const data = reactive(null)
   async function fetchData() {
     const res = await fetch('https://api.example.com/data')
     data = await res.json()
   }
   fetchData()
   </script>

   incorrectly tries to assign to a reactive object directly and calls fetchData immediately without lifecycle hook.

   <script setup>
   import { ref } from 'vue'
   const data = ref(null)
   fetch('https://api.example.com/data').then(res => {
     data = res.json()
   })
   </script>

   assigns data instead of data.value and lacks lifecycle hook.

   <script setup>
   import { ref, onMounted } from 'vue'
   const data = ref(null)
   onMounted(() => {
     fetch('https://api.example.com/data').then(res => res.json()).then(json => {
       data.value = json
     })
   })
   </script>

   uses promises correctly but

   <script setup>
   import { ref, onMounted } from 'vue'
   const data = ref(null)
   onMounted(async () => {
     const res = await fetch('https://api.example.com/data')
     data.value = await res.json()
   })
   </script>

   is cleaner and preferred with async/await.

4. Final Answer:

   Option A code snippet -> Option A

5. Quick Check:

   Use onMounted + ref + async/await = A [OK]

Hint: Use onMounted with async/await and ref for fetching data [OK]

Common Mistakes:

• Assigning directly to reactive instead of .value
• Not using lifecycle hooks for async fetch
• Mixing reactive and ref incorrectly