Watchers for side effects in Vue - Mini Project: Build & Apply

Using Watchers for Side Effects in Vue

📖 Scenario: You are building a simple Vue app that tracks a user's input text and automatically updates a message when the text changes.

🎯 Goal: Create a Vue component that uses a watch to react to changes in a reactive variable and update another variable as a side effect.

📋 What You'll Learn

Use Vue 3 Composition API with <script setup>

Create a reactive variable inputText initialized to an empty string

Create a reactive variable message initialized to 'Waiting for input...'

Use a watch to update message whenever inputText changes

Display both inputText and message in the template

💡 Why This Matters

🌍 Real World

Watching reactive data to trigger side effects is common in forms, live search, and dynamic UI updates.

💼 Career

Understanding watchers helps build responsive Vue apps that react to user input and data changes efficiently.

1

Set up reactive input variable

Inside the <script setup> block, import ref from 'vue' and create a reactive variable called inputText initialized to an empty string ''.

Vue

  
    
    Input: {{ inputText }}
    Message: {{ message }}
  



import { ref } from 'vue'

// Create reactive variable inputText initialized to empty string
// Your code here

Hint

Use ref('') to create a reactive string variable called inputText.

2

Add reactive message variable

Below the inputText declaration, create another reactive variable called message initialized to the string 'Waiting for input...'.

Vue

  
    
    Input: {{ inputText }}
    Message: {{ message }}
  



import { ref } from 'vue'

const inputText = ref('')
// Create reactive variable message initialized to 'Waiting for input...'
// Your code here

Hint

Use ref('Waiting for input...') to create message.

3

Add watcher for inputText changes

Import watch from 'vue'. Use watch to watch the inputText variable. Inside the watcher callback, update message.value to `You typed: ${inputText.value}` whenever inputText changes.

Vue

  
    
    Input: {{ inputText }}
    Message: {{ message }}
  



import { ref, watch } from 'vue'

const inputText = ref('')
const message = ref('Waiting for input...')

// Add a watch on inputText to update message.value
// Your code here

Hint

Use watch(inputText, () => { ... }) and update message.value inside.

4

Complete template bindings

Ensure the template uses v-model="inputText" on the input field, and displays {{ inputText }} and {{ message }} inside <p> tags as shown.

Vue

  
    
     tags -->
    
  



import { ref, watch } from 'vue'

const inputText = ref('')
const message = ref('Waiting for input...')

watch(inputText, () => {
  message.value = `You typed: ${inputText.value}`
})

Hint

Check that the input uses v-model="inputText" and both variables are shown in paragraphs.

Practice

1. What is the main purpose of a watch in Vue?

easy

A. To run code when reactive data changes

B. To define a new component

C. To style elements dynamically

D. To create a template for HTML

Solution

1. Step 1: Understand what watch does

   A watcher observes reactive data and triggers code when that data changes.

2. Step 2: Compare options to watcher purpose

   Only To run code when reactive data changes describes running code on reactive data change, which matches watcher behavior.

3. Final Answer:

   To run code when reactive data changes -> Option A

4. Quick Check:

   Watcher = run code on data change [OK]

Hint: Watchers react to data changes by running code [OK]

Common Mistakes:

• Confusing watchers with components
• Thinking watchers style elements
• Assuming watchers create templates

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

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

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

Solution

1. Step 1: Recall Vue 3 watch syntax for reactive refs

   In Vue 3 Composition API, to watch a ref's value, you typically pass a getter function like watch(() => count, callback) or watch(() => count.value, callback). Passing the ref directly watches the ref object, not its value.

2. Step 2: Check each option

   watch(count, ...) watches the ref object, not its value. watch({ count }, ...) passes an object, invalid. watch('count', ...) uses invalid string. watch(() => count, ...) watches the value returned by the function, which is the ref's value.

3. Final Answer:

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

4. Quick Check:

   Watch ref value with function source = watch(() => count, callback) [OK]

Hint: Pass a function returning the reactive value to watch [OK]

Common Mistakes:

• Passing { count } object instead of ref
• Using string 'count'
• Passing ref object directly instead of function

3. Given the code below, what will be logged when count.value changes from 1 to 2?

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. Changed from 1 to 2

B. Changed from 2 to 1

C. Changed from undefined to 2

D. No output

Solution

1. Step 1: Understand watcher callback parameters

   The watcher callback receives newVal and oldVal representing the new and previous values of the watched source.

2. Step 2: Trace the value change

   Initially, count.value is 1. When changed to 2, the watcher logs "Changed from 1 to 2".

3. Final Answer:

   Changed from 1 to 2 -> Option A

4. Quick Check:

   Old to new value logged = Changed from 1 to 2 [OK]

Hint: Watcher logs old and new values on change [OK]

Common Mistakes:

• Mixing old and new values order
• Expecting no output on value change
• Confusing initial value as undefined

4. Identify the error in this watcher code snippet:

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

medium

A. Callback function must have two parameters

B. Missing function to return count in watch source

C. Ref must be declared with reactive() instead of ref()

D. watch must be imported from 'vue-router'

Solution

1. Step 1: Check watch source argument

   In Vue 3 Composition API, to watch a ref, you must pass a function returning the ref's value, like watch(() => count.value, callback).

2. Step 2: Identify the error

   The code passes count.value directly, which is incorrect. It should be watch(() => count.value, ...).

3. Final Answer:

   Missing function to return count in watch source -> Option B

4. Quick Check:

   Watch source must be a function returning value [OK]

Hint: Watch source must be a function returning reactive value [OK]

Common Mistakes:

• Passing count.value (primitive) directly instead of ref or getter
• Thinking callback needs two parameters always
• Importing watch from wrong package

5. You want to watch a reactive object user and run a side effect only when the user.name changes, ignoring other properties. Which watcher setup is correct?

hard

A. watch(() => user, (newUser) => { console.log('User changed:', newUser) }, { deep: true })

B. watch(user, (newUser) => { console.log('User changed:', newUser) })

C. watch(() => user.name, (newName) => { console.log('Name changed:', newName) })

D. watch('user.name', (newName) => { console.log('Name changed:', newName) })

Solution

1. Step 1: Understand watching nested properties

   To watch a specific property like user.name, watch a function returning that property.

2. Step 2: Evaluate options

   watch(() => user.name, (newName) => { console.log('Name changed:', newName) }) watches user.name directly, triggering only on name changes. watch(user, (newUser) => { console.log('User changed:', newUser) }) watches whole object, triggering on any change. watch(() => user, (newUser) => { console.log('User changed:', newUser) }, { deep: true }) watches whole object deeply, also triggers on any change. watch('user.name', (newName) => { console.log('Name changed:', newName) }) uses a string, which is invalid in Vue 3 Composition API.

3. Final Answer:

   watch(() => user.name, (newName) => { console.log('Name changed:', newName) }) -> Option C

4. Quick Check:

   Watch specific property with function source = watch(() => user.name, (newName) => { console.log('Name changed:', newName) }) [OK]

Hint: Watch specific property with a function returning it [OK]

Common Mistakes:

• Watching whole object instead of property
• Using string source instead of function
• Using deep option unnecessarily