Reactivity caveats and limitations in Vue - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a reactive variable using Vue's Composition API.

Vue

import { ref } from 'vue';
const count = [1](0);

Drag options to blanks, or click blank then click option'

Aref

Breactive

Ccomputed

Dwatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using reactive instead of ref for primitive values

Using computed or watch incorrectly here

In Vue 3, ref is used to create a reactive primitive value.

2fill in blank

medium

Complete the code to correctly update a reactive object's property so Vue tracks the change.

Vue

import { reactive } from 'vue';
const state = reactive({ count: 0 });
state.[1] = 1;

Drag options to blanks, or click blank then click option'

Aupdate

Bvalue

Cset

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to use set or update methods which don't exist

Using .value which is for refs, not reactive objects

You update reactive object properties by assigning directly to the property name.

3fill in blank

hard

Fix the error in the code to make Vue track the new property added to a reactive object.

Vue

import { reactive } from 'vue';
const state = reactive({});
// Adding new property
state.[1] = 42;

Drag options to blanks, or click blank then click option'

AnewProp

Bvalue

Cset

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to use set or add methods which don't exist

Using .value which is for refs, not reactive objects

Vue 3's reactive can track new properties added directly, so just use the property name.

4fill in blank

hard

Fill both blanks to create a reactive array and add a new item so Vue tracks the change.

Vue

import { [1] } from 'vue';
const items = [2]([]);
items.value.push('apple');

Drag options to blanks, or click blank then click option'

Aref

Breactive

Ccomputed

Dwatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using reactive instead of ref for arrays

Trying to use computed or watch incorrectly

Use ref to make an array reactive and then modify it directly.

5fill in blank

hard

Fill all three blanks to create a reactive object, add a new property, and watch it for changes.

Vue

import { reactive, watch, [1] } from 'vue';
const state = reactive({ count: 0 });
state.[2] = 5;
watch(() => state.[3], (newVal) => console.log(newVal));

Drag options to blanks, or click blank then click option'

Aref

Btotal

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the new property and the watched property

Not importing ref when needed

Use ref in import, add property total, and watch total for changes.

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