Setup function basics in Vue - Performance & Optimization

Performance: Setup function basics

MEDIUM IMPACT

This affects the initial rendering speed and reactivity setup of Vue components.

Defining reactive state and computed properties inside the setup function

Vue

import { reactive, computed } from 'vue';
export default {
  setup() {
    const state = reactive({ count: 0 });
    const doubled = computed(() => state.count * 2);
    return { state, doubled };
  }
}

Simpler computed property avoids blocking initial render.

📈 Performance GainReduces blocking time to under 5ms

Defining reactive state and computed properties inside the setup function

Vue

import { reactive, computed } from 'vue';
export default {
  setup() {
    const state = reactive({ count: 0 });
    const doubled = computed(() => {
      // heavy computation inside computed
      let result = 0;
      for (let i = 0; i < 1000000; i++) {
        result += state.count * 2;
      }
      return result;
    });
    return { state, doubled };
  }
}

Heavy computation inside computed properties runs on initial render, blocking UI.

📉 Performance CostBlocks rendering for 50-100ms depending on device

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Heavy computation in setup	Minimal	0	High due to JS blocking	[X] Bad
Simple reactive state in setup	Minimal	0	Low	[OK] Good
Data fetching inside setup	Minimal	0	Blocks LCP until data arrives	[X] Bad
Data fetching in onMounted hook	Minimal	0	Non-blocking initial paint	[OK] Good

Rendering Pipeline

The setup function runs before the component renders. Heavy logic here delays style calculation and layout.

→JavaScript Execution

→Style Calculation

→Layout

⚠️ BottleneckJavaScript Execution blocking initial render

Core Web Vital Affected

LCP

This affects the initial rendering speed and reactivity setup of Vue components.

Optimization Tips

1Avoid heavy computations inside the setup function to prevent blocking initial render.

2Defer asynchronous tasks like data fetching to lifecycle hooks such as onMounted.

3Keep reactive state initialization simple and fast for better LCP.

Performance Quiz - 3 Questions

What is the main performance risk of doing heavy computations inside the setup function?

A It causes excessive DOM nodes to be created.

B It increases network requests.

C It blocks the initial rendering and delays Largest Contentful Paint.

D It causes layout shifts after render.

DevTools: Performance

How to check: Record a performance profile while loading the component and look for long scripting tasks during setup.

What to look for: Long 'Evaluate Script' tasks blocking the main thread before first paint indicate heavy setup logic.

Practice

1. What is the main purpose of the setup function in a Vue 3 component?

easy

A. To register global components

B. To define reactive data and functions before the component renders

C. To handle routing between pages

D. To style the component using CSS

Solution

1. Step 1: Understand the role of setup in Vue 3

   The setup function is where you declare reactive data and functions that the component will use.

2. Step 2: Compare with other options

   Styling, routing, and global registration are handled elsewhere, not in setup.

3. Final Answer:

   To define reactive data and functions before the component renders -> Option B

4. Quick Check:

   Setup function = reactive data + functions [OK]

Hint: Remember: setup is for data and logic, not styling or routing [OK]

Common Mistakes:

• Confusing setup with styling or routing
• Thinking setup runs after rendering
• Assuming setup registers global components

2. Which of the following is the correct way to declare a reactive variable inside the setup function using Vue 3 Composition API?

easy

A. const count = useState(0);

B. const count = reactive(0);

C. const count = reactive({ value: 0 });

D. const count = ref(0);

Solution

1. Step 1: Identify correct reactive declaration for simple values

   For simple reactive variables like numbers, ref is used, e.g., const count = ref(0);.

2. Step 2: Check other options

   reactive is for objects, not primitives; useState is not a Vue function.

3. Final Answer:

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

4. Quick Check:

   Simple reactive variable = ref() [OK]

Hint: Use ref() for single values, reactive() for objects [OK]

Common Mistakes:

• Using reactive() for primitive values
• Trying to use non-existent useState()
• Wrapping primitive in reactive({ value: ... }) unnecessarily

3. Consider this Vue 3 component setup code:

import { ref } from 'vue';

export default {
  setup() {
    const message = ref('Hello');
    function update() {
      message.value = 'Hi';
    }
    return { message, update };
  }
}

What will be the value of message after calling update()?

medium

A. 'Hi'

B. undefined

C. 'Hello'

D. An error occurs

Solution

1. Step 1: Understand ref and updating its value

   message is a ref holding 'Hello'. Calling update() sets message.value to 'Hi'.

2. Step 2: Determine final value after update

   After update(), message.value is 'Hi', so message reactive variable reflects 'Hi'.

3. Final Answer:

   'Hi' -> Option A

4. Quick Check:

   Ref value updated correctly = 'Hi' [OK]

Hint: Remember to access .value to update ref variables [OK]

Common Mistakes:

• Forgetting to use .value to update ref
• Assuming message itself changes, not message.value
• Thinking update() does not affect message

4. Identify the error in this Vue 3 setup function:

import { ref } from 'vue';

export default {
  setup() {
    const count = ref(0);
    function increment() {
      count = count + 1;
    }
    return { count, increment };
  }
}

medium

A. setup function should return a single object

B. Missing import of reactive

C. Cannot assign to ref variable directly; should use count.value

D. increment function is not declared correctly

Solution

1. Step 1: Check how ref variables are updated

   Ref variables hold their value inside .value. Direct assignment like count = count + 1 is invalid.

2. Step 2: Correct way to update ref

   Use count.value = count.value + 1 or count.value++ to update.

3. Final Answer:

   Cannot assign to ref variable directly; should use count.value -> Option C

4. Quick Check:

   Update ref with .value [OK]

Hint: Always update ref with .value, not direct assignment [OK]

Common Mistakes:

• Assigning directly to ref variable
• Forgetting to use .value when updating
• Confusing reactive and ref update syntax

5. You want to create a Vue 3 component using <script setup> that has a reactive object with properties name and age. Which code correctly defines this reactive object and updates age by 1 inside setup?

hard

A. import { reactive } from 'vue';

const person = reactive({ name: 'Alice', age: 30 });
person.age++;

B. import { ref } from 'vue';

const person = ref({ name: 'Alice', age: 30 });
person.age++;

C. import { reactive } from 'vue';

const person = reactive({ name: 'Alice', age: 30 });
person = { ...person, age: person.age + 1 };

D. import { ref } from 'vue';

const person = ref({ name: 'Alice', age: 30 });
person.value.age = person.age + 1;

Solution

1. Step 1: Choose correct reactive type for object

   Use reactive for objects to make all properties reactive.

2. Step 2: Update property correctly

   With reactive, update properties directly like person.age++. Using ref with objects requires accessing .value.

3. Final Answer:

   import { reactive } from 'vue'; const person = reactive({ name: 'Alice', age: 30 }); person.age++; -> Option A

4. Quick Check:

   Reactive object updated directly =

   import { reactive } from 'vue';
   
   const person = reactive({ name: 'Alice', age: 30 });
   person.age++;
   

   [OK]

Hint: Use reactive() for objects, update properties directly [OK]

Common Mistakes:

• Using ref for objects without .value access
• Reassigning reactive object instead of updating property
• Trying to increment property on ref object without .value