Ref and reactive in Composition API in Vue - Step-by-Step Execution

Concept Flow - Ref and reactive in Composition API

Start Setup

↓

Create ref or reactive

↓

Access or update value

↓

Vue tracks changes

↓

Component re-renders

↓

User sees updated UI

↓

Repeat updates as needed

This flow shows how Vue's ref and reactive create reactive state, track changes, and update the UI automatically.

Execution Sample

Vue

import { ref, reactive } from 'vue'

const count = ref(0)
const state = reactive({ name: 'Vue' })

count.value++
state.name = 'Vue 3'

This code creates a ref and a reactive object, then updates their values to trigger reactivity.

Execution Table

Step	Variable	Action	Value Before	Value After	Reactivity Triggered	UI Update
1	count	Initialize ref	undefined	0	No	No
2	state	Initialize reactive	undefined	{ name: 'Vue' }	No	No
3	count.value	Increment	0	1	Yes	Yes - count displayed updates
4	state.name	Change property	'Vue'	'Vue 3'	Yes	Yes - name displayed updates

💡 All reactive changes processed, UI updated accordingly

Variable Tracker

Variable	Start	After Step 3	After Step 4
count.value	0	1	1
state.name	'Vue'	'Vue'	'Vue 3'

Key Moments - 2 Insights

Why do we use count.value to access the ref but state.name directly for reactive?

Refs wrap the value inside a .value property, so you must use count.value (see step 3). Reactive objects are proxies, so you access properties directly like state.name (step 4).

Does changing count.value or state.name immediately update the UI?

Yes, Vue tracks these changes and triggers UI updates automatically after the change (see 'Reactivity Triggered' and 'UI Update' columns in steps 3 and 4).

Visual Quiz - 3 Questions

Look at the execution table, what is count.value after step 3?

A 0

B undefined

C 1

D 2

💡 Hint

Check the 'Value After' column for count.value at step 3 in the execution table.

At which step does the UI update to show the new state.name value?

A Step 1

B Step 4

C Step 2

D Step 3

💡 Hint

Look at the 'UI Update' column for state.name changes in the execution table.

If we forget to use .value with count, what happens?

A No update happens because .value is required

B The value updates correctly

C Vue throws an error immediately

D Reactive works instead of ref

💡 Hint

Recall that refs require .value to access or update the inner value (see key moments).

Concept Snapshot

Vue Composition API uses ref() to create a reactive single value accessed via .value.
Use reactive() to create a reactive object accessed directly.
Changing ref.value or reactive properties triggers automatic UI updates.
Always use .value with refs, but not with reactive objects.
This enables simple, tracked state changes in components.

Full Transcript

In Vue's Composition API, ref and reactive create reactive state. Ref wraps a single value accessed with .value, while reactive wraps an object accessed directly. When you change these values, Vue tracks the changes and updates the UI automatically. For example, incrementing count.value or changing state.name triggers re-rendering. Remember to use .value with refs, or changes won't be tracked. This flow helps you build dynamic, responsive interfaces easily.

Practice

1. In Vue's Composition API, what is the main difference between ref and reactive?

easy

A. ref is used to create components, reactive is used to create directives.

B. ref is only for arrays, reactive is only for strings.

C. ref is for simple values accessed with .value, reactive is for objects and arrays accessed directly.

D. ref and reactive are exactly the same and can be used interchangeably.

Solution

1. Step 1: Understand ref usage

   ref holds simple reactive values like numbers or strings and requires accessing the value with .value.

2. Step 2: Understand reactive usage

   reactive wraps objects or arrays to make their properties reactive and accessed directly without .value.

3. Final Answer:

   ref is for simple values accessed with .value, reactive is for objects and arrays accessed directly. -> Option C

4. Quick Check:

   ref vs reactive difference = D [OK]

Hint: Remember: ref = simple value with .value, reactive = object direct access [OK]

Common Mistakes:

• Thinking ref works directly on objects without .value
• Confusing reactive as only for primitive values
• Assuming ref and reactive are interchangeable

2. Which of the following is the correct way to declare a reactive object in Vue 3 Composition API?

easy

A. const state = ref({ count: 0 })

B. const state = reactive({ count: 0 })

C. const state = reactive(0)

D. const state = ref(0)

Solution

1. Step 1: Identify correct reactive syntax for objects

   reactive is used to wrap objects to make their properties reactive.

2. Step 2: Check each option

   const state = ref({ count: 0 }) uses ref with an object, which is allowed but requires .value to access. const state = reactive({ count: 0 }) correctly uses reactive with an object. const state = reactive(0) uses reactive with a number, which is invalid. const state = ref(0) uses ref with a number, which is valid but not for objects.

3. Final Answer:

   const state = reactive({ count: 0 }) -> Option B

4. Quick Check:

   reactive object syntax = C [OK]

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

Common Mistakes:

• Using reactive() on primitive values
• Using ref() for objects without .value access
• Confusing reactive and ref usage

3. Given the code:

import { ref, reactive } from 'vue';
const count = ref(0);
const state = reactive({ clicks: 0 });

count.value++;
state.clicks++;

console.log(count.value, state.clicks);

What will be logged to the console?

medium

A. 0 1

B. 0 0

C. 1 0

D. 1 1

Solution

1. Step 1: Understand ref and reactive increments

   count is a ref holding 0, incremented by count.value++ to 1. state is reactive object with clicks 0, incremented by state.clicks++ to 1.

2. Step 2: Check console.log output

   Logging count.value and state.clicks will print 1 1.

3. Final Answer:

   1 1 -> Option D

4. Quick Check:

   Increment ref and reactive values = A [OK]

Hint: Increment ref with .value, reactive directly [OK]

Common Mistakes:

• Forgetting to use .value with ref
• Assuming reactive needs .value
• Mixing up initial values

4. What is wrong with this Vue Composition API code?

import { ref } from 'vue';
const user = ref({ name: 'Alice' });
user.name = 'Bob';

medium

A. You must use user.value.name to update the name property.

B. ref cannot hold objects, use reactive instead.

C. You should use user.value() to access the object.

D. There is no error; the code works fine.

Solution

1. Step 1: Understand ref with objects

   ref wraps the object, so to access or update properties inside, you must use .value.

2. Step 2: Identify the error in property update

   The code tries to update user.name directly, which is incorrect. It should be user.value.name.

3. Final Answer:

   You must use user.value.name to update the name property. -> Option A

4. Quick Check:

   Access ref object properties with .value = A [OK]

Hint: Use .value to access or update ref object properties [OK]

Common Mistakes:

• Accessing ref object properties without .value
• Confusing ref and reactive usage
• Thinking ref cannot hold objects

5. You want to create a reactive state for a form with fields name (string) and tags (array). You also want to track a simple counter for submissions. Which setup correctly uses ref and reactive together?

hard

A. const form = reactive({ name: '', tags: [] }); const submitCount = ref(0);

B. const form = ref({ name: '', tags: [] }); const submitCount = reactive(0);

C. const form = reactive(''); const submitCount = ref([]);

D. const form = ref(''); const submitCount = reactive({ count: 0 });

Solution

1. Step 1: Identify correct usage for form object

   The form has multiple fields, so reactive is best to wrap the object with name and tags properties.

2. Step 2: Identify correct usage for simple counter

   The submission count is a simple number, so ref is appropriate to track it reactively.

3. Final Answer:

   const form = reactive({ name: '', tags: [] }); const submitCount = ref(0); -> Option A

4. Quick Check:

   Use reactive for objects, ref for simple values = B [OK]

Hint: Use reactive for objects/arrays, ref for simple values [OK]

Common Mistakes:

• Using ref for complex objects without .value
• Using reactive on primitive values
• Mixing up which data needs ref or reactive