Watch vs computed decision in Vue - Quick Revision & Key Differences

Recall & Review

beginner

What is a computed property in Vue?

A computed property is a reactive value that Vue caches and updates only when its dependencies change. It is used to calculate values based on reactive data and is efficient because it avoids unnecessary recalculations.

beginner

What is a watch in Vue used for?

A watch is used to perform side effects or run code in response to changes in reactive data. It is useful for asynchronous operations or when you want to react to data changes without directly computing a value.

intermediate

When should you choose computed over watch?

Choose computed when you want to derive a value from reactive data that updates automatically and is cached for performance. It is best for simple, synchronous calculations.

intermediate

When is watch the better choice than computed?

Use watch when you need to perform side effects like API calls, logging, or asynchronous tasks triggered by data changes. It is also useful when you want to react to changes without returning a value.

intermediate

How does Vue optimize computed properties compared to methods?

Vue caches computed properties and only recalculates them when their dependencies change. Methods run every time they are called, so computed is more efficient for derived data.

Which Vue feature caches its result until dependencies change?

A watch

B computed

C methods

D props

computed properties cache their results and update only when dependencies change, improving performance.

Which Vue feature is best for running asynchronous code when data changes?

A computed

B methods

C props

D watch

watch is designed to react to data changes and run side effects like async calls.

If you want to derive a value from reactive data without side effects, you should use:

A computed

B methods

C watch

D emit

computed is ideal for deriving values reactively and efficiently.

Which Vue feature runs code every time it is called, without caching?

A computed

B watch

C methods

D props

methods run fresh every time they are called, unlike computed properties.

What is a common use case for watch in Vue?

A Performing an API call when data changes

B Passing props to child components

C Rendering HTML

D Calculating a value from data

watch is often used to trigger side effects like API calls when reactive data changes.

Explain the difference between watch and computed in Vue and when to use each.

Think about caching and side effects.

Describe a scenario where using watch is better than computed in a Vue app.

Consider tasks that happen outside of just returning a 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