Watch vs computed decision in Vue - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a computed property in Vue.

Vue

<script setup>
import { ref, [1] } from 'vue'
const count = ref(0)
const double = [1](() => count.value * 2)
</script>

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

Acomputed

Breactive

Cwatch

Dref

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'watch' instead of 'computed' for reactive values.

Forgetting to import 'computed' from Vue.

The computed function creates a reactive computed property that updates automatically when its dependencies change.

2fill in blank

medium

Complete the code to watch the 'count' ref and log its new value.

Vue

<script setup>
import { ref, watch } from 'vue'
const count = ref(0)
watch(count, (newVal) => {
  console.log([1])
})
</script>

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

AoldVal

Bcount.value

CnewVal

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Logging the ref itself instead of its new value.

Using 'count' instead of the callback parameter.

The watcher callback receives the new value as the first argument, commonly named newVal.

3fill in blank

hard

Fix the error in the watch usage to correctly watch a nested property.

Vue

<script setup>
import { reactive, watch } from 'vue'
const state = reactive({ user: { name: 'Alice' } })
watch([1], (newName) => {
  console.log('Name changed to', newName)
})
</script>

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

A() => state.user.name

Bstate.user.name

Cstate.user

Dstate

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the nested property directly instead of a function.

Watching the whole object instead of the specific property.

To watch a nested property in a reactive object, use a getter function like () => state.user.name.

4fill in blank

hard

Fill both blanks to create a computed property that depends on 'firstName' and 'lastName'.

Vue

<script setup>
import { ref, [1] } from 'vue'
const firstName = ref('John')
const lastName = ref('Doe')
const fullName = [2](() => `${firstName.value} ${lastName.value}`)
</script>

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

Acomputed

Bwatch

Creactive

Dref

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'watch' instead of 'computed' for the property.

Importing 'ref' instead of 'computed'.

Both blanks require computed to import and create the computed property.

5fill in blank

hard

Fill both blanks to watch 'age' and update 'isAdult' accordingly.

Vue

<script setup>
import { ref, watch } from 'vue'
const age = ref(17)
const isAdult = ref(false)
watch([1], (newAge) => {
  isAdult.value = newAge [2] 18
})
</script>

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

Aage

B>=

C<

Dage.value

Attempts:

3 left

💡 Hint

Common Mistakes

Watching 'age.value' instead of 'age'.

Using '<' instead of '>=' for the adult check.

Watch the age ref, compare newAge with 18 using '>=' operator, and assign the result to isAdult.value.

Practice

1. Which Vue feature should you use to create a value that automatically updates and caches based on other reactive data?

easy

A. Computed property

B. Watch property

C. Method

D. Lifecycle hook

Solution

1. Step 1: Understand computed properties

   Computed properties automatically update when their dependencies change and cache their results for efficiency.

2. Step 2: Compare with watch

   Watch is for running code on data changes, not for caching reactive values.

3. Final Answer:

   Computed property -> Option A

4. Quick Check:

   Reactive cached value = Computed property [OK]

Hint: Use computed for reactive cached values, watch for side effects [OK]

Common Mistakes:

• Using watch to compute and cache values
• Confusing methods with computed properties
• Thinking lifecycle hooks handle reactive caching

2. Which of the following is the correct syntax to define a computed property named fullName in Vue 3 Composition API using computed?

easy

A. const fullName = reactive(() => firstName + ' ' + lastName);

B. const fullName = watch(() => firstName + ' ' + lastName);

C. const fullName = computed(() => firstName + ' ' + lastName);

D. const fullName = ref(() => firstName + ' ' + lastName);

Solution

1. Step 1: Identify correct function for computed

   The computed function takes a getter function and returns a reactive, cached value.

2. Step 2: Check other options

   watch is for side effects, reactive creates reactive objects, ref creates reactive primitives, none are for computed caching.

3. Final Answer:

   const fullName = computed(() => firstName + ' ' + lastName); -> Option C

4. Quick Check:

   Computed syntax = computed(() => ...) [OK]

Hint: Use computed(() => ...) for reactive cached values [OK]

Common Mistakes:

• Using watch instead of computed for caching
• Confusing reactive and ref with computed
• Missing arrow function inside computed

3. Given the Vue 3 setup below, what will be logged when count.value changes from 0 to 1?

const count = ref(0);
watch(count, (newVal, oldVal) => {
  console.log(`Count changed from ${oldVal} to ${newVal}`);
});
count.value = 1;

medium

A. Error: watch callback parameters incorrect

B. Count changed from 1 to 0

C. No output

D. Count changed from 0 to 1

Solution

1. Step 1: Understand watch behavior

   The watch function runs its callback when the watched reactive value changes, passing new and old values.

2. Step 2: Analyze the code

   When count.value changes from 0 to 1, the callback logs the change with oldVal=0 and newVal=1.

3. Final Answer:

   Count changed from 0 to 1 -> Option D

4. Quick Check:

   Watch logs old and new values on change [OK]

Hint: Watch logs old and new values on reactive changes [OK]

Common Mistakes:

• Mixing newVal and oldVal order
• Expecting watch to cache values like computed
• Not calling .value on ref when changing

4. Identify the error in this Vue 3 watch usage:

const count = ref(0);
watch(() => count, (newVal, oldVal) => {
  console.log(newVal);
});
count.value = 5;

medium

A. The watch source should be count not a function returning count

B. The watch source function should return count.value instead of count

C. The watch callback parameters are reversed

D. No error, code works correctly

Solution

1. Step 1: Understand watch source function

   The watch source function should return the reactive value to watch, not the ref object itself.

2. Step 2: Identify the mistake

   The function returns count (a ref), but it should return count.value to watch the actual value.

3. Final Answer:

   The watch source function should return count.value instead of count -> Option B

4. Quick Check:

   Watch source must return reactive value, not ref object [OK]

Hint: Return .value inside watch source function to track value changes [OK]

Common Mistakes:

• Returning ref instead of its value in watch source
• Confusing parameter order in watch callback
• Assuming watch works without reactive source

5. You want to update the document title whenever the user's full name changes, which is computed from first and last names. Which Vue approach is best to implement this behavior?

hard

A. Use a computed property for full name and a watch on full name to update document title

B. Use only a computed property for full name and update document title inside it

C. Use a watch on first and last names separately to update document title

D. Use a method to compute full name and call it inside watch to update document title

Solution

1. Step 1: Separate display logic and side effects

   Computed properties are ideal for reactive display values like full name, caching the result.

2. Step 2: Use watch for side effects

   Updating document title is a side effect, so watch the computed full name to react to changes.

3. Final Answer:

   Use a computed property for full name and a watch on full name to update document title -> Option A

4. Quick Check:

   Computed for values, watch for side effects [OK]

Hint: Compute values with computed, react with watch for side effects [OK]

Common Mistakes:

• Trying to update document title inside computed (not allowed)
• Watching first and last names separately instead of computed
• Using methods instead of computed for reactive values