Typing ref and reactive in Vue - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a typed ref for a number.

Vue

const count = ref<[1]>(0);

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

Anumber

Bobject

Cboolean

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using string instead of number for a numeric ref.

Forgetting to add the type inside < >.

We use ref<number> to tell Vue this ref holds a number.

2fill in blank

medium

Complete the code to create a reactive object with a typed property.

Vue

const state = reactive({ name: [1]('') });

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

Areactive

Bcomputed

Cref

Dwatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using reactive instead of ref for a single property.

Using computed or watch incorrectly here.

Use ref inside reactive to create a reactive property.

3fill in blank

hard

Fix the error in typing a reactive object with a string property.

Vue

const user = reactive<{ name: [1] }>({ name: 'Alice' });

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

Astring

Bboolean

Cnumber

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Typing name as number or boolean incorrectly.

Using any without reason.

The property name is a string, so type it as string.

4fill in blank

hard

Fill both blanks to type a ref holding an array of numbers.

Vue

const numbers = ref<[1]<[2]>>([]);

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

AArray

Bnumber

Cstring

DSet

Attempts:

3 left

💡 Hint

Common Mistakes

Using string instead of number for array items.

Using Set instead of Array here.

Use Array<number> to type an array of numbers inside a ref.

5fill in blank

hard

Fill all three blanks to type a reactive object with a boolean and a string property.

Vue

const settings = reactive<{ [1]: [2], [3]: string }>({ darkMode: true, language: 'en' });

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

AdarkMode

Bboolean

Clanguage

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing property names or types incorrectly.

Using number instead of boolean for darkMode.

The object has a boolean property darkMode and a string property language.

Practice

1. Which Vue function should you use to create a reactive variable holding a simple number with type safety?

easy

A. ref<number>(0)

B. reactive<number>({ value: 0 })

C. ref({ count: 0 })

D. reactive(0)

Solution

1. Step 1: Understand the use of ref for simple values

   ref<Type> is used to create reactive primitive values like numbers or strings with type safety.

2. Step 2: Recognize the correct syntax for a typed ref

   The syntax ref<number>(0) correctly creates a reactive number initialized to 0.

3. Final Answer:

   ref<number>(0) -> Option A

4. Quick Check:

   Simple reactive value uses ref<Type> = D [OK]

Hint: Use ref<Type> for simple values, reactive<Type> for objects [OK]

Common Mistakes:

• Using reactive for simple primitive values
• Omitting the type parameter in ref
• Using ref with an object instead of reactive

2. Which of the following is the correct syntax to create a reactive object with type safety in Vue 3?

easy

A. const state = ref<{ count: number }>({ count: 0 })

B. const state = reactive<{ count: number }>({ count: 0 })

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

D. const state = ref({ count: 0 })

Solution

1. Step 1: Identify the correct function for reactive objects

   Use reactive<Type> to create reactive objects with type safety.

2. Step 2: Check the syntax correctness

   const state = reactive<{ count: number }>({ count: 0 }) correctly types the object and makes it reactive.

3. Final Answer:

   const state = reactive<{ count: number }>({ count: 0 }) -> Option B

4. Quick Check:

   Reactive object with type uses reactive<Type> = A [OK]

Hint: Use reactive<Type> for objects, ref<Type> for primitives [OK]

Common Mistakes:

• Using ref instead of reactive for objects
• Missing type parameter in reactive
• Using reactive without an object

3. Given the code:

const count = ref<number>(5);
count.value = 10;
console.log(count.value);

What will be logged to the console?

medium

A. 5

B. undefined

C. 10

D. Error: count is not a function

Solution

1. Step 1: Understand ref and .value usage

   ref<number>(5) creates a reactive number with initial value 5. To update or read the value, use .value.

2. Step 2: Follow the code execution

   Setting count.value = 10 updates the reactive value. Logging count.value outputs 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   ref value updated and logged = 10 [OK]

Hint: Access ref values with .value to get or set [OK]

Common Mistakes:

• Logging ref directly without .value
• Confusing reactive object access with ref
• Expecting initial value after update

4. What is wrong with this Vue 3 code snippet?

const state = reactive<{ count: number }>({ count: 0 });
state.count = 'one';

medium

A. Runtime error: reactive objects are immutable

B. Syntax error: reactive cannot be typed

C. No error, code works fine

D. Type error: assigning string to number property

Solution

1. Step 1: Check the declared type of reactive object

   The object is typed as { count: number }, so count must be a number.

2. Step 2: Analyze the assignment

   Assigning 'one' (a string) to count violates the type, causing a type error.

3. Final Answer:

   Type error: assigning string to number property -> Option D

4. Quick Check:

   Type mismatch in reactive object property = C [OK]

Hint: Check property types before assigning new values [OK]

Common Mistakes:

• Ignoring type errors in reactive objects
• Thinking reactive objects are immutable
• Confusing syntax errors with type errors

5. You want to create a reactive state in Vue 3 that holds a list of user objects with typed properties id: number and name: string. Which is the best way to type and initialize this state?

hard

A. const users = ref<Array<{ id: number; name: string }>>([])

B. const users = reactive<Array<{ id: number; name: string }>>({})

C. const users = reactive<{ id: number; name: string }>([])

D. const users = ref<{ id: number; name: string }>([])

Solution

1. Step 1: Decide between ref and reactive for arrays

   Arrays are objects, but Vue recommends using ref for arrays to keep reactivity simple and clear.

2. Step 2: Apply correct typing for array of objects

   Typing as Array<{ id: number; name: string }> ensures each user object has correct properties.

3. Step 3: Check initialization syntax

   ref<Array<{ id: number; name: string }>>([]) initializes an empty reactive array with proper typing.

4. Final Answer:

   const users = ref<Array<{ id: number; name: string }>>([]) -> Option A

5. Quick Check:

   Typed reactive array uses ref with array type = A [OK]

Hint: Use ref<Array<Type>> for reactive arrays of objects [OK]

Common Mistakes:

• Using reactive with array type incorrectly
• Typing reactive as single object instead of array
• Using ref with object type for array