Ref and reactive in Composition API in Vue

Introduction

Ref and reactive help you keep track of data that changes in your Vue app. They make your app update automatically when data changes.

When you want to store a simple value like a number or string that changes over time.

When you need to keep an object or array reactive so Vue updates the UI when it changes.

When building components that need to respond to user input or events.

When you want to share reactive data between functions inside the setup() function.

When you want Vue to watch and react to changes in your data automatically.

Syntax

Vue

import { ref, reactive } from 'vue';

const count = ref(0);
const user = reactive({ name: 'Alice', age: 25 });

ref is for simple values like numbers, strings, or booleans.

reactive is for objects and arrays to make all their properties reactive.

Examples

This creates a reactive string value. Use message.value to get or set it.

Vue

const message = ref('Hello');

This creates a reactive object. You can access properties like user.name.

Vue

const user = reactive({ name: 'Bob', age: 30 });

Update the value inside a ref by changing its .value property.

Vue

message.value = 'Hi there!';

Update a property inside a reactive object directly.

Vue

user.age = 31;

Sample Program

This Vue component uses ref for a simple number and reactive for an object. Buttons update the values, and the UI updates automatically.

Vue

<script setup>
import { ref, reactive } from 'vue';

const count = ref(0);
const user = reactive({ name: 'Emma', age: 28 });

function increment() {
  count.value++;
}

function haveBirthday() {
  user.age++;
}
</script>

<template>
  <div>
    <p>Count: {{ count }}</p>
    <button @click="increment">Increase Count</button>

    <p>User: {{ user.name }}, Age: {{ user.age }}</p>
    <button @click="haveBirthday">Have Birthday</button>
  </div>
</template>

Important Notes

Always use .value to read or write a ref value inside JavaScript.

For reactive objects, access and update properties directly without .value.

Use ref for simple data and reactive for complex objects or arrays.

Summary

ref holds simple reactive values accessed with .value.

reactive makes objects and arrays reactive with direct property access.

Both help Vue update the UI automatically when data changes.

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