Setup function basics in Vue

Introduction

The setup function is where you prepare your component's data and logic in Vue 3. It helps organize your code clearly and makes it easy to use Vue's new features.

When creating a new Vue 3 component using the Composition API.

When you want to organize reactive data and functions in one place.

When you need to use Vue's reactivity system like ref or reactive.

When you want to write cleaner and more reusable component logic.

Syntax

Vue

<script setup>
import { ref } from 'vue'
// Define reactive data
const count = ref(0)

// Define a function
function increment() {
  count.value++
}
</script>

<template>
  <button @click="increment">Count is: {{ count }}</button>
</template>

The <script setup> block is a special Vue 3 syntax that runs the setup function automatically.

Use ref() to create reactive variables that update the UI when changed.

Examples

Simple reactive string displayed in the template.

Vue

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

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

Button that increases count when clicked, showing reactive updates.

Vue

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

<template>
  <button @click="increment">Clicked {{ count }} times</button>
</template>

Using reactive to track an object and update age on button click.

Vue

<script setup>
import { reactive } from 'vue'
const user = reactive({ name: 'Anna', age: 25 })
function birthday() {
  user.age++
}
</script>

<template>
  <p>{{ user.name }} is {{ user.age }} years old.</p>
  <button @click="birthday">Happy Birthday!</button>
</template>

Sample Program

This component shows a button with a count. Each click increases the count number. The ref makes the count reactive, so the UI updates automatically.

It also includes an aria-label for accessibility, helping screen readers describe the button.

Vue

<script setup>
import { ref } from 'vue'

const count = ref(0)

function increment() {
  count.value++
}
</script>

<template>
  <button @click="increment" aria-label="Increment count">Count is: {{ count }}</button>
</template>

Important Notes

Always import ref or reactive from 'vue' to create reactive data.

Use value to access or change the value inside a ref.

The <script setup> block is simpler and recommended for new Vue 3 components.

Summary

The setup function is where you define reactive data and functions in Vue 3.

Use ref for simple reactive variables and reactive for objects.

<script setup> makes writing setup code easier and cleaner.

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