Watch and watchEffect in Vue - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the correct function for watching reactive data.

Vue

import { [1] } from 'vue';

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

Aref

Breactive

Ccomputed

Dwatch

Attempts:

3 left

💡 Hint

Common Mistakes

Importing 'ref' instead of 'watch'.

Using 'reactive' which creates reactive objects but does not watch.

The watch function is used to watch reactive data changes in Vue.

2fill in blank

medium

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

Vue

watch(count, (newVal, oldVal) => { console.log([1]); });

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

AnewVal

BoldVal

Ccount

Dthis.count

Attempts:

3 left

💡 Hint

Common Mistakes

Logging the old value instead of the new value.

Trying to access 'this.count' inside the callback.

The callback receives the new value as the first argument, which we log.

3fill in blank

hard

Fix the error in the watchEffect usage to correctly log the reactive 'message'.

Vue

watchEffect(() => { console.log([1]); });

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

Amessage.value

Bmessage

Cthis.message

Dmessage()

Attempts:

3 left

💡 Hint

Common Mistakes

Logging the ref object instead of its value.

Trying to call the ref as a function.

When watching a ref inside watchEffect, you must access its value with .value.

4fill in blank

hard

Fill both blanks to create a watch that triggers only when 'user.name' changes and logs the new name.

Vue

watch(() => user[1]name, (newName) => { console.log(newName[2]); });

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

A.

B[

C]

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Using brackets instead of dot for property access.

Forgetting parentheses when calling console.log.

The first blank needs a dot to access the property. The second blank uses parentheses to call the function console.log.

5fill in blank

hard

Fill all three blanks to create a watchEffect that updates 'fullName' by combining 'firstName' and 'lastName'.

Vue

watchEffect(() => { fullName.value = [1] + ' ' + [2]; [3] });

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

AfirstName.value

BlastName.value

Cconsole.log(fullName.value)

DfullName

Attempts:

3 left

💡 Hint

Common Mistakes

Using the ref names without .value.

Forgetting to log the updated fullName.

Access the values of firstName and lastName refs with .value. Then log the updated fullName.

Practice

1. What is the main difference between watch and watchEffect in Vue?

easy

A. watchEffect only runs once, watch runs continuously.

B. watch tracks specific reactive sources, watchEffect tracks all reactive dependencies automatically.

C. watch runs immediately on setup, watchEffect waits for changes.

D. watchEffect requires manual cleanup, watch does not.

Solution

1. Step 1: Understand watch behavior

   watch listens to specific reactive data sources you specify and runs a callback only when those change.

2. Step 2: Understand watchEffect behavior

   watchEffect runs immediately and tracks all reactive data accessed inside it, rerunning whenever any of those change.

3. Final Answer:

   watch tracks specific reactive sources, watchEffect tracks all reactive dependencies automatically. -> Option B

4. Quick Check:

   watch vs watchEffect difference = watch tracks specific reactive sources, watchEffect tracks all reactive dependencies automatically. [OK]

Hint: Remember: watch tracks specific data, watchEffect tracks all used data [OK]

Common Mistakes:

• Thinking watchEffect only runs once
• Assuming watch runs immediately on setup
• Confusing cleanup requirements

2. Which of the following is the correct syntax to watch a reactive variable count in Vue 3 Composition API?

easy

A. watch(count, (newVal, oldVal) => { console.log(newVal) })

B. watch(count, () => { console.log(count) })

C. watchEffect(() => { console.log(count) })

D. watch(() => count, (newVal, oldVal) => { console.log(newVal) })

Solution

1. Step 1: Identify correct watch syntax

   In Vue 3 Composition API, watch accepts a ref directly as the first argument, which automatically watches its .value.

2. Step 2: Check callback parameters

   The callback receives newVal and oldVal as parameters, correctly used in watch(count, (newVal, oldVal) => { console.log(newVal) }) to log the new value.

3. Final Answer:

   watch(count, (newVal, oldVal) => { console.log(newVal) }) -> Option A

4. Quick Check:

   Correct watch syntax uses ref directly = watch(count, (newVal, oldVal) => { console.log(newVal) }) [OK]

Hint: Pass ref directly to watch [OK]

Common Mistakes:

• Using getter returning ref object like () => count
• Confusing watch with watchEffect syntax
• Omitting callback parameters

3. Given the code below, what will be logged to the console after count.value = 2 is executed?

import { ref, watch } from 'vue'
const count = ref(1)
watch(() => count.value, (newVal, oldVal) => {
  console.log(`Changed from ${oldVal} to ${newVal}`)
})
count.value = 2

medium

A. SyntaxError

B. Changed from undefined to 2

C. No output

D. Changed from 1 to 2

Solution

1. Step 1: Understand watch callback parameters

   The watch callback receives old and new values of the watched reactive source.

2. Step 2: Trace the change

   Initially, count.value is 1. When changed to 2, the callback logs the change from 1 to 2.

3. Final Answer:

   Changed from 1 to 2 -> Option D

4. Quick Check:

   watch logs old and new values correctly = Changed from 1 to 2 [OK]

Hint: watch callback shows old and new values on change [OK]

Common Mistakes:

• Expecting no output on change
• Confusing initial value with undefined
• Misunderstanding callback parameters

4. What is wrong with this Vue 3 code using watch? How to fix it?

import { ref, watch } from 'vue'
const count = ref(0)
watch(count, (newVal) => {
  console.log(newVal)
})

medium

A. No error; code works fine

B. Missing cleanup function; fix by adding cleanup inside callback

C. Callback missing oldVal parameter; fix by adding it

D. Passing reactive variable directly; fix by passing a getter function like () => count

Solution

1. Step 1: Identify watch argument type

   In Vue 3, watch(count, callback) is valid; it automatically tracks count.value changes.

2. Step 2: Callback parameters

   The callback receives newVal and optional oldVal; omitting oldVal is fine.

3. Final Answer:

   No error; code works fine -> Option A

4. Quick Check:

   watch accepts ref directly = No error; code works fine [OK]

Hint: watch accepts refs directly [OK]

Common Mistakes:

• Thinking ref must be wrapped in getter
• Assuming oldVal always required
• Believing code has error

5. You want to run a side effect immediately and re-run it whenever any reactive data inside changes, but also want to stop the effect manually later. Which Vue 3 API and pattern should you use?

hard

A. Use watch without immediate option and no cleanup

B. Use watch with immediate: true option and no stop function

C. Use watchEffect and store the returned stop function to call later

D. Use watchEffect but it cannot be stopped manually

Solution

1. Step 1: Identify immediate reactive effect

   watchEffect runs immediately and tracks all reactive dependencies automatically.

2. Step 2: Understand stopping effect

   watchEffect returns a stop function that can be called to stop the effect manually.

3. Final Answer:

   Use watchEffect and store the returned stop function to call later -> Option C

4. Quick Check:

   watchEffect runs immediately and can be stopped = Use watchEffect and store the returned stop function to call later [OK]

Hint: watchEffect returns stop function to control effect lifecycle [OK]

Common Mistakes:

• Thinking watchEffect cannot be stopped
• Using watch without immediate for immediate run
• Ignoring stop function usage