Why Readonly for immutable reactive data in Vue? - Purpose & Use Cases

The Big Idea

What if you could share data safely without worrying about accidental changes breaking your app?

The Scenario

Imagine you have a shared shopping list app where multiple parts of your code can change the list. You want some parts to only see the list without changing it.

The Problem

Manually tracking who can change the list is tricky. If someone accidentally changes it, bugs appear and data gets messy. It's hard to keep everything in sync and safe.

The Solution

Readonly wrappers in Vue make data immutable for certain parts of your app. They let you share data safely without worrying about accidental changes.

Before vs After

✗ Before

const list = reactive(['apple', 'banana'])
// No protection, any code can change list

✓ After

const list = reactive(['apple', 'banana'])
const readonlyList = readonly(list)
// readonlyList cannot be changed accidentally

What It Enables

This lets you confidently share data across your app, knowing some parts can only read it, preventing bugs and keeping state predictable.

Real Life Example

In a dashboard, the main data updates live, but some widgets only display it without changing. Using readonly ensures widgets don't accidentally modify the data.

Key Takeaways

Manual data protection is error-prone and hard to maintain.

Readonly creates safe, immutable views of reactive data.

This improves app stability and prevents accidental bugs.

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