v-memo for conditional memoization in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is v-memo in Vue?

v-memo is a directive in Vue that helps Vue remember (memoize) parts of the template. It skips re-rendering those parts if the specified conditions (dependencies) haven't changed.

beginner

How does v-memo improve performance?

By telling Vue to skip re-rendering parts of the template when certain data hasn't changed, v-memo reduces unnecessary work and speeds up updates.

intermediate

What kind of values do you pass to v-memo?

You pass an array of reactive values or expressions that v-memo watches. If none of these values change, Vue skips re-rendering the memoized part.

beginner

Example: How to use v-memo to memoize a list item that depends on item.name and item.count?

<li v-memo="[item.name, item.count]">{{ item.name }}: {{ item.count }}</li>

This tells Vue to only re-render the <li> if item.name or item.count changes.

intermediate

When should you NOT use v-memo?

Avoid using v-memo if the dependencies are complex or change often, because the overhead of checking might outweigh the benefits. Also, don’t use it if the part is cheap to render.

What does v-memo do in Vue?

A Skips re-rendering when specified dependencies don't change

B Forces re-rendering every time

C Removes elements from the DOM

D Creates a new component instance

v-memo helps Vue skip re-rendering parts of the template if the dependencies stay the same.

Which of these is a valid use of v-memo?

A &lt;div v-memo="count + user.name"&gt;Hello&lt;/div&gt;

B &lt;div v-memo="true"&gt;Hello&lt;/div&gt;

C &lt;div v-memo&gt;Hello&lt;/div&gt;

D &lt;div v-memo="[count, user.name]"&gt;Hello&lt;/div&gt;

v-memo expects an array of dependencies to watch for changes.

If none of the dependencies in v-memo change, what happens?

A Vue throws an error

B Vue skips re-rendering the memoized part

C Vue re-renders the memoized part anyway

D Vue removes the memoized part from the DOM

The main purpose of v-memo is to skip re-rendering when dependencies are unchanged.

Which situation is NOT ideal for using v-memo?

A When you want to optimize performance

B When rendering is expensive

C When dependencies change very often

D When dependencies rarely change

If dependencies change often, the cost of checking may outweigh benefits.

What type of values should you include in the v-memo array?

A Reactive values that affect rendering

B Static strings only

C Functions only

D DOM elements

Only reactive values that influence the rendered output should be included.

Explain how v-memo helps Vue optimize rendering and when you should use it.

Think about how Vue decides to update parts of the page.

Describe how to use v-memo in a Vue template with an example.

Show the syntax and what it means for rendering.

Practice

1. What is the main purpose of v-memo in Vue?

easy

A. To skip re-rendering parts of the template when watched values don't change

B. To force Vue to always re-render a component

C. To create reactive data properties

D. To handle user input events

Solution

1. Step 1: Understand what v-memo does

   v-memo watches specific values and skips re-rendering if they remain unchanged.

2. Step 2: Compare options with this behavior

   Only To skip re-rendering parts of the template when watched values don't change describes skipping re-rendering based on watched values, matching v-memo's purpose.

3. Final Answer:

   To skip re-rendering parts of the template when watched values don't change -> Option A

4. Quick Check:

   v-memo skips re-rendering [OK]

Hint: Remember: v-memo skips updates if values unchanged [OK]

Common Mistakes:

• Confusing v-memo with event handling
• Thinking v-memo creates reactive data
• Assuming v-memo forces re-rendering

2. Which of the following is the correct syntax to use v-memo watching variables count and user.name?

easy

A. <div v-memo="[count, user.name]">...

B. <div v-memo="count, user.name">...

C. <div v-memo="{count, user.name}">...

D. <div v-memo="count && user.name">...

Solution

1. Step 1: Recall the syntax for v-memo

   v-memo expects an array of values to watch, so the correct syntax uses square brackets.

2. Step 2: Check each option's syntax

   <div v-memo="[count, user.name]">... uses an array with square brackets, which is correct. Options A, B, and C use incorrect formats (logical AND, comma-separated without brackets, object).

3. Final Answer:

   <div v-memo="[count, user.name]">... -> Option A

4. Quick Check:

   v-memo needs array syntax [OK]

Hint: Use square brackets for v-memo watched values [OK]

Common Mistakes:

• Omitting brackets around watched values
• Using object braces instead of array brackets
• Using logical operators instead of array

3. Given this Vue template snippet:

<template>
  <div v-memo="[count]">Count is {{ count }}</div>
</template>

<script setup>
import { ref } from 'vue'
const count = ref(0)

setTimeout(() => { count.value = 0 }, 1000)
</script>

What will happen after 1 second?

medium

A. The div will re-render but display the old count

B. The div will re-render and update the displayed count

C. Vue will throw a runtime error due to v-memo usage

D. The div will not re-render because count did not change

Solution

1. Step 1: Understand the watched value and update

   The v-memo watches count. After 1 second, count.value is set to 0, which is the same as its initial value.

2. Step 2: Determine if re-render occurs

   Since the watched value did not change, v-memo skips re-rendering the div.

3. Final Answer:

   The div will not re-render because count did not change -> Option D

4. Quick Check:

   v-memo skips if values unchanged [OK]

Hint: If watched values stay same, v-memo skips update [OK]

Common Mistakes:

• Assuming re-render happens even if value unchanged
• Confusing ref value assignment with new value
• Thinking v-memo causes errors on no change

4. Identify the error in this usage of v-memo:

<template>
  <div v-memo="count">Count: {{ count }}</div>
</template>

<script setup>
import { ref } from 'vue'
const count = ref(0)
</script>

medium

A. The template must use v-if with v-memo

B. count should be a computed property, not a ref

C. v-memo expects an array, but count is not in brackets

D. There is no error; this is valid usage

Solution

1. Step 1: Check v-memo syntax requirements

   v-memo requires an array of values to watch, so the value must be inside square brackets.

2. Step 2: Analyze the given code

   The code uses v-memo="count" without brackets, which is incorrect syntax.

3. Final Answer:

   v-memo expects an array, but count is not in brackets -> Option C

4. Quick Check:

   v-memo needs array syntax [OK]

Hint: Always wrap watched values in brackets for v-memo [OK]

Common Mistakes:

• Using single variable without brackets
• Confusing refs with computed properties
• Thinking v-memo requires v-if

5. You want to optimize a list rendering in Vue where each item has a status and value. You only want to re-render an item if its status changes, ignoring value. How should you use v-memo inside the v-for loop?

hard

A. <li v-for="item in items" :key="item.id" v-memo="[item.value]">{{ item.value }}</li>

B. <li v-for="item in items" :key="item.id" v-memo="[item.status]">{{ item.value }}</li>

C. <li v-for="item in items" :key="item.id" v-memo="[item.status, item.value]">{{ item.value }}</li>

D. <li v-for="item in items" :key="item.id">{{ item.value }}</li> (no v-memo needed)

Solution

1. Step 1: Identify which value controls re-rendering

   You want to re-render only when item.status changes, ignoring item.value.

2. Step 2: Use v-memo with the correct watched value

   Setting v-memo="[item.status]" watches only the status, so changes in value won't trigger re-render.

3. Final Answer:

   <li v-for="item in items" :key="item.id" v-memo="[item.status]">{{ item.value }}</li> -> Option B

4. Quick Check:

   Watch only status for re-render [OK]

Hint: Watch only needed props in v-memo to optimize updates [OK]

Common Mistakes:

• Watching value instead of status
• Watching both status and value unnecessarily
• Not using v-memo and causing extra renders