Script setup syntax in Vue - Performance & Optimization

Performance: Script setup syntax

MEDIUM IMPACT

This affects the initial component compilation and runtime performance by simplifying component setup and reducing boilerplate code.

Defining component logic in Vue

Vue

<script setup>
import { ref } from 'vue';
const count = ref(0);
function increment() {
  count.value++;
}
</script>

Script setup compiles to more efficient code with less runtime overhead and smaller bundle size.

📈 Performance GainSaves ~2-3kb in bundle and speeds up component mount time

Defining component logic in Vue

Vue

export default {
  data() {
    return { count: 0 };
  },
  methods: {
    increment() {
      this.count++;
    }
  }
}

This traditional options API adds extra runtime overhead and larger bundle size due to verbose structure.

📉 Performance CostAdds ~2-3kb more to bundle and triggers slower component initialization

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Options API	No difference	No difference	No difference	[!] OK
Script setup syntax	No difference	No difference	No difference	[OK] Good

Rendering Pipeline

Script setup syntax compiles Vue component logic at build time, reducing runtime parsing and setup cost.

→Compile

→JavaScript Execution

→Component Initialization

⚠️ BottleneckComponent Initialization

Core Web Vital Affected

LCP

This affects the initial component compilation and runtime performance by simplifying component setup and reducing boilerplate code.

Optimization Tips

1Use script setup to reduce component runtime overhead.

2Script setup syntax compiles logic at build time, improving load speed.

3Avoid options API for better bundle size and initialization performance.

Performance Quiz - 3 Questions

How does using Vue's script setup syntax affect bundle size compared to the options API?

A It increases bundle size due to extra imports

B It has no effect on bundle size

C It reduces bundle size by removing runtime overhead

D It doubles the bundle size

DevTools: Performance

How to check: Record a performance profile while loading the component and look at scripting and scripting evaluation times.

What to look for: Lower scripting time and faster component setup indicate better performance with script setup syntax.

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