Reactivity caveats and limitations in Vue - Mini Project: Build & Apply

Understanding Reactivity Caveats and Limitations in Vue

📖 Scenario: You are building a simple Vue 3 app to track a list of tasks. You want to learn how Vue's reactivity system works and what its limits are.This will help you avoid common mistakes when updating data so your app updates correctly.

🎯 Goal: Create a Vue 3 component that shows a list of tasks and lets you add new tasks. You will learn how to set up reactive data, add a reactive property, update the list reactively, and handle Vue's reactivity caveats.

📋 What You'll Learn

Use Vue 3 Composition API with <script setup>

Create a reactive array of tasks

Add a reactive property to track new task input

Add a function to add new tasks to the reactive array

Demonstrate a reactivity caveat by adding a new property to an existing reactive object

💡 Why This Matters

🌍 Real World

Understanding Vue's reactivity caveats helps you build reliable user interfaces that update correctly when data changes.

💼 Career

Vue developers must know how reactivity works to avoid bugs and write maintainable, efficient code in real projects.

1

Setup reactive tasks array

Create a Vue 3 component with <script setup>. Inside it, create a reactive array called tasks with these exact strings: 'Buy groceries', 'Walk the dog', and 'Read a book'.

Vue

  
    Task List
  



// Create a reactive array called tasks with 'Buy groceries', 'Walk the dog', and 'Read a book'

Hint

Use ref from Vue to create a reactive array called tasks.

2

Add reactive new task input

Add a reactive string variable called newTask initialized to an empty string '' inside the <script setup>.

Vue

  
    Task List
  



import { ref } from 'vue'

const tasks = ref(['Buy groceries', 'Walk the dog', 'Read a book'])
// Create a reactive string variable newTask initialized to ''

Hint

Use ref to create a reactive string variable newTask with an empty string.

3

Add function to add new tasks

Add a function called addTask that pushes the value of newTask.value into the tasks.value array, then clears newTask.value to an empty string ''.

Vue

  
    Task List
  



import { ref } from 'vue'

const tasks = ref(['Buy groceries', 'Walk the dog', 'Read a book'])
const newTask = ref('')

// Add a function addTask that pushes newTask.value to tasks.value and clears newTask.value

Hint

Define a function addTask that updates the reactive array and clears the input.

4

Demonstrate reactivity caveat with new property

Inside <script setup>, create a reactive object called taskDetails with one property completed set to false. Then add a new property priority with value 'high' directly on taskDetails.value after creation to show Vue does not detect this addition reactively.

Vue

  
    Task List
  



import { ref } from 'vue'

const tasks = ref(['Buy groceries', 'Walk the dog', 'Read a book'])
const newTask = ref('')

function addTask() {
  tasks.value.push(newTask.value)
  newTask.value = ''
}

// Create reactive object taskDetails with completed: false
// Then add priority: 'high' property directly on taskDetails.value

Hint

Create a reactive object with ref and add a new property after creation to see Vue's reactivity caveat.

Practice

1. Which of the following is a known limitation of Vue's reactivity system?

easy

A. Using Vue's reactive() function disables reactivity for nested objects.

B. Changing a reactive property always triggers an update immediately.

C. Adding a new property directly to an existing reactive object won't trigger updates.

D. Vue automatically tracks changes to all array indices without exceptions.

Solution

1. Step 1: Understand Vue's reactivity on object properties

   Vue cannot detect new properties added directly to an existing reactive object after its creation.

2. Step 2: Compare options with this limitation

   Adding a new property directly to an existing reactive object won't trigger updates correctly states this limitation. The other options are incorrect because reactive() supports nested objects, property changes do trigger updates (though batched asynchronously), and Vue has exceptions for some array changes like setting length.

3. Final Answer:

   Adding a new property directly to an existing reactive object won't trigger updates. -> Option C

4. Quick Check:

   New object properties not reactive = C [OK]

Hint: Remember: Vue can't detect new object properties added later [OK]

Common Mistakes:

• Assuming all object property additions are reactive
• Thinking array index changes always trigger updates
• Believing reactive() disables nested reactivity

2. Which syntax correctly updates an array element reactively in Vue?

easy

A. myArray[2] = 'new value'

B. myArray = ['new value']

C. myArray.push('new value')

D. myArray.$set(2, 'new value')

Solution

1. Step 1: Identify reactive array update methods

   Directly assigning to an array index (like myArray[2] = ...) is reactive in Vue 3 for reactive arrays.

2. Step 2: Evaluate each option

   myArray[2] = 'new value' uses direct assignment which triggers reactivity in Vue 3. myArray = ['new value'] replaces the whole array. myArray.push('new value') adds an element at the end. myArray.$set(2, 'new value') uses Vue 2 syntax unavailable in Vue 3.

3. Final Answer:

   myArray[2] = 'new value' -> Option A

4. Quick Check:

   Direct array index updates reactive in Vue 3 = A [OK]

Hint: Directly assign to reactive array indices in Vue 3 [OK]

Common Mistakes:

• Replacing the entire array instead of updating an element
• Using push() to update a specific index
• Using Vue 2 methods like $set in Vue 3

3. Given the code:

const state = reactive({ items: ['a', 'b', 'c'] });
state.items[1] = 'x';
console.log(state.items);

What will be logged?

medium

A. ['a', 'b', 'c']

B. ['a', 'x', 'c']

C. Error: Cannot assign to reactive array index

D. ['a', 'b', 'c', 'x']

Solution

1. Step 1: Understand array index reactivity in Vue 3

   In Vue 3, reactive arrays track direct index assignments reactively, so state.items[1] = 'x' updates the array.

2. Step 2: Predict the console output

   The array changes from ['a', 'b', 'c'] to ['a', 'x', 'c'], so logging state.items shows the updated array.

3. Final Answer:

   ['a', 'x', 'c'] -> Option B

4. Quick Check:

   Vue 3 reactive arrays track index changes = A [OK]

Hint: Vue 3 tracks direct array index changes reactively [OK]

Common Mistakes:

• Thinking direct index assignment is not reactive in Vue 3
• Expecting an error on index assignment
• Confusing array push with index update

4. What is the problem with this Vue reactive code?

const obj = reactive({});
obj.newProp = 123;
console.log(obj.newProp);

medium

A. newProp is added but changes won't trigger updates

B. newProp is reactive and logs 123

C. newProp is not reactive and logs undefined

D. Code throws a runtime error

Solution

1. Step 1: Check adding new properties to reactive objects

   Vue cannot detect new properties added directly to reactive objects after creation, so newProp is added but not reactive.

2. Step 2: Understand console output and reactivity

   newProp logs 123 correctly, but changes to it won't trigger UI updates because Vue didn't track it reactively.

3. Final Answer:

   newProp is added but changes won't trigger updates -> Option A

4. Quick Check:

   New object properties added later not tracked = B [OK]

Hint: New object properties added later are not reactive by default [OK]

Common Mistakes:

• Assuming newProp is reactive immediately
• Expecting undefined instead of 123
• Thinking code throws an error

5. You want to add a new property age to a reactive object user and ensure Vue tracks it reactively. Which approach is correct?

hard

A. user['age'] = 30

B. user.age = 30

C. Vue.set(user, 'age', 30)

D. user = { ...user, age: 30 }

Solution

1. Step 1: Understand Vue 3 reactivity and adding new properties

   Vue 3 does not track new properties added directly to reactive objects. Using object spread creates a new reactive object with the new property.

2. Step 2: Evaluate each option for reactivity

   Direct property assignments do not make properties reactive. Vue.set(user, 'age', 30) is Vue 2 syntax and not available in Vue 3. user = { ...user, age: 30 } creates a new reactive object with age included, ensuring reactivity.

3. Final Answer:

   user = { ...user, age: 30 } -> Option D

4. Quick Check:

   Use object spread to add reactive properties = A [OK]

Hint: Use object spread to add reactive properties in Vue 3 [OK]

Common Mistakes:

• Using Vue.set in Vue 3 (deprecated)
• Adding properties directly expecting reactivity
• Confusing direct assignment with reactive update