Setup function basics in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the setup() function in Vue 3?

The setup() function is where you write the logic for a Vue component using the Composition API. It runs before the component is created and returns the data and methods that the template can use.

beginner

How do you return reactive data from the setup() function?

You return an object containing reactive references (like ref or reactive) or plain values. These become available in the component's template.

beginner

Which Vue 3 API is commonly used inside setup() to create a reactive variable?

The ref() function is used to create a reactive variable that can hold a single value and update the template when changed.

intermediate

Can you access component properties like props and context inside setup()?

Yes, setup() receives props and context as arguments, allowing you to use props and emit events or access slots.

intermediate

Why is setup() considered better for organizing code than the Options API?

Because setup() groups related logic together, making code easier to read and maintain, especially in larger components.

What does the setup() function return in a Vue component?

A Nothing, it only runs code

B A string with HTML content

C A Vue instance

D An object with data and methods for the template

The setup() function returns an object whose properties are exposed to the template for rendering and interaction.

Which of these is a reactive reference you can create inside setup()?

A ref()

B reactiveVar()

C watch()

D computed()

ref() creates a reactive reference holding a value that updates the template when changed.

How do you access component props inside setup()?

A Using <code>props()</code> function

B Using <code>this.props</code>

C They are passed as the first argument to <code>setup()</code>

D Props are not accessible in <code>setup()</code>

Props are passed as the first argument to setup(props, context).

Which Vue 3 feature does setup() belong to?

A Composition API

B Options API

C Vue Router

D Vuex

setup() is the entry point for the Composition API in Vue 3.

What is the benefit of using setup() over Options API?

A It runs faster in the browser

B Better code organization by grouping related logic

C It automatically styles components

D It replaces the need for templates

setup() helps organize code by logic rather than by option type, improving readability.

Explain how the setup() function works in a Vue 3 component and what it returns.

Think about how you prepare data and methods for the template.

Describe how to create and use a reactive variable inside the setup() function.

Remember how Vue tracks changes to update the UI.

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