Watch vs computed decision in Vue

Introduction

We use computed properties to automatically calculate values based on other data. We use watch to run code when data changes, like reacting to changes with side effects.

When you want to show a value that updates automatically based on other data.

When you need to perform an action or side effect after some data changes.

When you want to cache a value that depends on reactive data for better performance.

When you want to watch a specific data property and run code only when it changes.

When you want to transform data for display without extra code.

Syntax

Vue

import { ref, computed, watch } from 'vue';

const count = ref(0);

// Computed property
const double = computed(() => count.value * 2);

// Watcher
watch(count, (newVal, oldVal) => {
  console.log(`Count changed from ${oldVal} to ${newVal}`);
});

computed returns a reactive value that updates automatically.

watch runs a function when the watched data changes, useful for side effects.

Examples

This computed property converts the message to uppercase automatically.

Vue

const message = ref('Hello');
const shout = computed(() => message.value.toUpperCase());

This watcher logs a message every time the message changes.

Vue

watch(message, (newVal, oldVal) => {
  console.log(`Message changed from ${oldVal} to ${newVal}`);
});

Computed calculates doubleCount, watcher alerts when count passes 10.

Vue

const count = ref(0);
const doubleCount = computed(() => count.value * 2);

watch(count, (newVal) => {
  if (newVal > 10) {
    alert('Count is greater than 10!');
  }
});

Sample Program

This Vue component shows a count and its double. Clicking the button increases count. The computed property updates doubleCount automatically. The watcher logs changes to count in the console.

Vue

<template>
  <div>
    <p>Count: {{ count }}</p>
    <p>Double: {{ doubleCount }}</p>
    <button @click="increment">Increment</button>
  </div>
</template>

<script setup>
import { ref, computed, watch } from 'vue';

const count = ref(0);

const doubleCount = computed(() => count.value * 2);

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

function increment() {
  count.value++;
}
</script>

Important Notes

Use computed when you want to derive and display data reactively.

Use watch when you want to perform side effects like API calls or logging after data changes.

Computed properties cache their results until dependencies change, making them efficient.

Summary

Computed is for reactive, cached values based on other data.

Watch is for running code when data changes, especially side effects.

Choose computed for display logic, watch for reacting to changes.

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