Readonly for immutable reactive data in Vue - Mini Project: Build & Apply

Readonly for immutable reactive data

📖 Scenario: You are building a Vue 3 app that manages user profile data. Some parts of the data should not be changed by child components to avoid accidental updates.

🎯 Goal: Create a reactive user profile object and make it immutable using Vue's readonly function. Then pass this readonly data to a child component to ensure it cannot be modified.

📋 What You'll Learn

Create a reactive object called userProfile with keys name and age and values 'Alice' and 30

Create a readonly version of userProfile called immutableProfile

Use immutableProfile in the template to display the user's name and age

Pass immutableProfile as a prop to a child component called UserCard

💡 Why This Matters

🌍 Real World

In real apps, some data should be protected from changes by child components to avoid bugs. Using readonly helps enforce this immutability.

💼 Career

Understanding reactive and readonly data is essential for Vue developers to manage state safely and build maintainable applications.

1

Create reactive user profile data

Create a reactive object called userProfile with keys name set to 'Alice' and age set to 30 using Vue's reactive function.

Vue

import { reactive } from 'vue'

// Create reactive userProfile object
// Your code here



  

Hint

Use const userProfile = reactive({ name: 'Alice', age: 30 }) to create the reactive object.

2

Create readonly immutable profile

Import Vue's readonly function and create a readonly version of userProfile called immutableProfile.

Vue

import { reactive, readonly } from 'vue'

const userProfile = reactive({ name: 'Alice', age: 30 })

// Create readonly immutableProfile from userProfile
// Your code here



  

Hint

Use const immutableProfile = readonly(userProfile) to create the immutable reactive object.

3

Display immutable profile data in template

Use immutableProfile.name and immutableProfile.age inside the template to display the user's name and age in paragraphs.

Vue

import { reactive, readonly } from 'vue'

const userProfile = reactive({ name: 'Alice', age: 30 })
const immutableProfile = readonly(userProfile)



  
  

Hint

Use mustache syntax like {{ immutableProfile.name }} inside paragraph tags.

4

Pass immutable profile to child component

Create a child component called UserCard and pass immutableProfile as a prop named profile to it in the template.

Vue

import { reactive, readonly } from 'vue'
import UserCard from './UserCard.vue'

const userProfile = reactive({ name: 'Alice', age: 30 })
const immutableProfile = readonly(userProfile)



  Name: {{ immutableProfile.name }}
  Age: {{ immutableProfile.age }}
  
  

Hint

Use <UserCard :profile="immutableProfile" /> to pass the readonly data as a prop.

Practice

1. What does the readonly function do in Vue's reactivity system?

easy

A. It creates an immutable reactive object that cannot be changed directly.

B. It makes a reactive object completely non-reactive and static.

C. It clones the reactive object and allows changes only on the clone.

D. It converts a reactive object into a plain JavaScript object.

Solution

1. Step 1: Understand the purpose of readonly

   The readonly function wraps a reactive object to prevent direct modifications, making it immutable.

2. Step 2: Compare with other behaviors

   Unlike cloning or removing reactivity, readonly keeps the object reactive but disallows changes.

3. Final Answer:

   It creates an immutable reactive object that cannot be changed directly. -> Option A

4. Quick Check:

   Readonly = Immutable reactive object [OK]

Hint: Readonly means no direct changes allowed on reactive data [OK]

Common Mistakes:

• Thinking readonly removes reactivity completely
• Assuming readonly clones data instead of wrapping
• Believing readonly allows changes on the wrapped object

2. Which of the following is the correct way to create a readonly reactive object in Vue?

easy

A. const data = readonly({ count: 0 })

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

C. const data = reactive(readonly({ count: 0 }))

D. const data = reactive({ count: 0 }).readonly()

Solution

1. Step 1: Understand the correct usage order

   You first create a reactive object, then wrap it with readonly to make it immutable.

2. Step 2: Evaluate each option

   const data = readonly(reactive({ count: 0 })) correctly wraps a reactive object with readonly. const data = reactive(readonly({ count: 0 })) tries to make a readonly object reactive, which is invalid. const data = readonly({ count: 0 }) skips reactive, so no reactivity. const data = reactive({ count: 0 }).readonly() uses a non-existent method.

3. Final Answer:

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

4. Quick Check:

   Wrap reactive with readonly = const data = readonly(reactive({ count: 0 })) [OK]

Hint: Wrap reactive() inside readonly() to make immutable reactive [OK]

Common Mistakes:

• Calling readonly before reactive
• Skipping reactive and only using readonly
• Using non-existent readonly method on reactive

3. Given the code below, what will be logged to the console?

const state = reactive({ count: 1 });
const readOnlyState = readonly(state);

readOnlyState.count = 5;
console.log(readOnlyState.count);
state.count = 10;
console.log(readOnlyState.count);

medium

A. Error on assignment, then 10

B. 5 and 10

C. 1 and 1

D. 1 and 10

Solution

1. Step 1: Understand readonly behavior on assignment

   Assigning to readOnlyState.count causes a warning in development mode, but the change is ignored and value remains 1.

2. Step 2: Check reactive source update reflection

   First console.log(readOnlyState.count) outputs 1. Changing state.count to 10 updates the source, so second log is 10.

3. Final Answer:

   1 and 10 -> Option D

4. Quick Check:

   Readonly warns/ignores direct change, reflects source updates [OK]

Hint: Readonly blocks direct changes but shows source updates [OK]

Common Mistakes:

• Expecting readonly to allow direct assignment
• Thinking readonly copies data instead of reflecting source
• Ignoring error/warning on assignment to readonly

4. Identify the error in the following Vue code using readonly:

import { reactive, readonly } from 'vue';
const data = readonly({ count: 0 });
data.count = 1;
console.log(data.count);

medium

A. You cannot assign to a readonly object directly.

B. You must use ref() instead of reactive() for readonly.

C. Readonly does not exist in Vue's API.

D. You must wrap a reactive object with readonly, not a plain object.

Solution

1. Step 1: Check how readonly is used

   The code creates a readonly proxy but then attempts data.count = 1.

2. Step 2: Understand readonly requirements

   Readonly prevents direct mutations, causing a development warning and ignoring the assignment.

3. Final Answer:

   You cannot assign to a readonly object directly. -> Option A

4. Quick Check:

   Cannot mutate readonly objects [OK]

Hint: Readonly objects cannot be mutated directly [OK]

Common Mistakes:

• Attempting to mutate readonly objects directly
• Assuming readonly works without reactive
• Confusing readonly with ref

5. You have a reactive object state shared across components. You want to expose it as readonly to prevent accidental changes but still reflect updates. Which approach correctly achieves this and why?

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

// Options:
// A) const safeState = readonly(state);
// B) const safeState = reactive(state);
// C) const safeState = readonly({ ...state });
// D) const safeState = reactive(readonly(state));

hard

A. Create a shallow copy and make it readonly, but it won't reflect nested updates.

B. Wrap the reactive state with readonly to prevent direct changes but reflect updates.

C. Wrap the state again with reactive, which allows changes and duplicates reactivity.

D. Wrap readonly with reactive, which is invalid and causes errors.

Solution

1. Step 1: Analyze each option's effect

   Wrap the reactive state with readonly to prevent direct changes but reflect updates. correctly wraps the reactive object with readonly, making it immutable but still reactive to source changes. Wrap the state again with reactive, which allows changes and duplicates reactivity. duplicates reactivity and allows changes. Create a shallow copy and make it readonly, but it won't reflect nested updates. creates a shallow copy, losing nested reactivity. Wrap readonly with reactive, which is invalid and causes errors. is invalid usage.

2. Step 2: Confirm correct approach for safe sharing

   Readonly wrapping reactive is the recommended way to share data safely without allowing modifications but reflecting updates.

3. Final Answer:

   Wrap the reactive state with readonly to prevent direct changes but reflect updates. -> Option C

4. Quick Check:

   Readonly wraps reactive for safe, immutable sharing [OK]

Hint: Wrap reactive with readonly to share immutable reactive data [OK]

Common Mistakes:

• Using shallow copies losing nested reactivity
• Wrapping readonly inside reactive incorrectly
• Duplicating reactivity causing unexpected mutations