Reactive Map and Set in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean for a Map or Set to be reactive in Vue?

A reactive Map or Set in Vue automatically updates the UI or any dependent code when its contents change, just like reactive objects or arrays.

beginner

How do you create a reactive Map in Vue 3.4+?

Use reactive(new Map()) to create a reactive Map that Vue tracks for changes.

beginner

How do you add an item to a reactive Set in Vue?

Use the Set's add() method on the reactive Set, for example: mySet.add(value). Vue will track this change.

intermediate

Why should you avoid replacing a reactive Map or Set with a new instance?

Replacing a reactive Map or Set with a new instance breaks reactivity because Vue tracks the original object. Instead, modify the existing Map or Set.

intermediate

How can you watch changes in a reactive Map or Set in Vue?

Use Vue's watch() function on the reactive Map or Set. It tracks changes like additions or deletions and runs your callback.

How do you make a Map reactive in Vue 3.4+?

A ref(new Map())

B computed(() => new Map())

C reactive(new Map())

D shallowReactive(new Map())

Using reactive(new Map()) creates a fully reactive Map that Vue tracks for changes.

Which method adds an item to a reactive Set in Vue?

A push()

B add()

C set()

D append()

Sets use the add() method to add items.

What happens if you replace a reactive Map with a new Map instance?

A Vue merges both Maps

B Vue tracks both Maps

C Reactivity continues normally

D Reactivity breaks for the new Map

Replacing the reactive Map breaks reactivity because Vue tracks the original object only.

How can you detect changes in a reactive Set?

A Using watch() on the Set

B Using computed() on the Set

C Using ref() on the Set

D Using toRaw() on the Set

watch() tracks changes like additions or deletions in reactive Sets.

Which Vue function creates a reactive object that tracks Map and Set changes?

A reactive()

B ref()

C readonly()

D shallowRef()

reactive() creates deep reactivity, including for Map and Set.

Explain how Vue makes a Map or Set reactive and why this is useful.

Think about how Vue tracks changes and updates the interface.

Describe how you would watch for changes in a reactive Set and respond to them.

Consider Vue's watch API and how it works with reactive collections.

Practice

1. Which Vue function should you use to make a Map reactive so that UI updates when it changes?

easy

A. computed()

B. ref()

C. watch()

D. reactive()

Solution

1. Step 1: Understand Vue reactivity basics

   Vue's reactive() wraps objects like Map or Set to track changes.

2. Step 2: Identify correct function for Map reactivity

   Only reactive() can wrap Map to make it reactive, so UI updates on mutations.

3. Final Answer:

   reactive() -> Option D

4. Quick Check:

   Wrap Map with reactive() = C [OK]

Hint: Use reactive() to wrap Map or Set for reactivity [OK]

Common Mistakes:

• Using ref() which works for primitives, not Map/Set
• Confusing computed() with reactive()
• Trying watch() to make Map reactive

2. Which of the following is the correct syntax to create a reactive Set in Vue?

easy

A. const mySet = reactive(new Set())

B. const mySet = ref(new Set())

C. const mySet = reactive(Set())

D. const mySet = reactive([])

Solution

1. Step 1: Check how to create reactive Set

   You must create a new Set instance and wrap it with reactive().

2. Step 2: Identify correct syntax

   reactive(new Set()) is correct; Set() without new is invalid, and ref() wraps primitives.

3. Final Answer:

   const mySet = reactive(new Set()) -> Option A

4. Quick Check:

   Use new Set() inside reactive() = B [OK]

Hint: Always use new Set() inside reactive() for reactive Sets [OK]

Common Mistakes:

• Omitting new keyword for Set
• Using ref() instead of reactive() for Set
• Wrapping an array instead of Set

3. Given this Vue setup:

const myMap = reactive(new Map());
myMap.set('a', 1);
myMap.set('b', 2);
myMap.delete('a');
console.log([...myMap.keys()]);

What will be logged to the console?

medium

A. ['b']

B. ['a', 'b']

C. []

D. ['a']

Solution

1. Step 1: Track Map mutations

   Initially, keys 'a' and 'b' are added. Then 'a' is deleted.

2. Step 2: Determine final keys

   After deletion, only 'b' remains as a key in the Map.

3. Final Answer:

   ['b'] -> Option A

4. Quick Check:

   Deleted 'a', keys left = ['b'] [OK]

Hint: Deleting a key removes it from Map keys list [OK]

Common Mistakes:

• Forgetting the delete call removes 'a'
• Assuming keys remain unchanged after mutation
• Confusing keys() with values()

4. What is wrong with this Vue code that tries to make a reactive Map?

const myMap = reactive(Map());
myMap.set('key', 'value');

medium

A. reactive() cannot wrap Map

B. Map() must be called with new keyword

C. set() method does not exist on Map

D. Missing ref() wrapper

Solution

1. Step 1: Check Map instantiation syntax

   Map must be created with new Map(), not just Map().

2. Step 2: Identify error cause

   Calling Map() without new causes a TypeError or unexpected behavior.

3. Final Answer:

   Map() must be called with new keyword -> Option B

4. Quick Check:

   Use new Map() to create Map instance [OK]

Hint: Always use new Map() to create Map instances [OK]

Common Mistakes:

• Omitting new keyword for Map
• Thinking reactive() can't wrap Map
• Using ref() instead of reactive()

5. You have a reactive Set in Vue:

const mySet = reactive(new Set(['apple', 'banana']));

// Later in code
mySet.add('banana');
mySet.delete('apple');

const items = [...mySet];

What will be the value of items after these operations?

hard

A. ['apple', 'banana', 'banana']

B. ['banana', 'apple']

C. ['banana']

D. ['apple']

Solution

1. Step 1: Understand Set behavior with duplicates

   Sets only store unique values, so adding 'banana' again does nothing.

2. Step 2: Track mutations

   Initially contains 'apple' and 'banana'. After deleting 'apple', only 'banana' remains.

3. Final Answer:

   ['banana'] -> Option C

4. Quick Check:

   Set keeps unique items, deleted 'apple' = ['banana'] [OK]

Hint: Sets ignore duplicate adds and remove items on delete [OK]

Common Mistakes:

• Assuming duplicates appear in Set
• Forgetting delete removes item
• Confusing Set with Array behavior