Keep-alive for expensive components in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the component in Vue?

The component caches inactive components to avoid re-rendering them, improving performance for expensive components.

beginner

How does improve user experience with expensive components?

By caching components, lets users switch back without waiting for the component to reload or fetch data again.

intermediate

Which Vue lifecycle hooks are called when a component is activated or deactivated inside ?

The activated() hook runs when the component is inserted back, and deactivated() runs when it is cached but hidden.

intermediate

How can you control which components caches?

Use the include and exclude props with component names or regex patterns to specify which components to cache or ignore.

beginner

What happens if you don’t use for expensive components?

The component will be destroyed and recreated every time it is switched, causing delays and repeated data fetching.

What does the component do in Vue?

A Removes components from the DOM permanently

B Caches inactive components to improve performance

C Automatically updates component data

D Prevents components from rendering

caches inactive components so they don’t get destroyed, speeding up switching.

Which lifecycle hook runs when a cached component becomes active again inside ?

A created()

B mounted()

C activated()

D deactivated()

activated() runs when a cached component is inserted back into the DOM.

How can you specify which components should cache?

A Using the include and exclude props

B Using the v-if directive

C Using the v-show directive

D Using the ref attribute

include and exclude props let you control caching by component name or pattern.

What is a common use case for ?

A Styling components dynamically

B Loading external scripts

C Handling user input events

D Caching expensive components like charts or forms

is great for caching components that take time to load or render.

What happens to a component inside when it is not active?

A It remains cached but hidden

B It reloads data automatically

C It is destroyed and removed from memory

D It becomes read-only

Inactive components inside stay cached in memory but are hidden from view.

Explain how works in Vue and why it is useful for expensive components.

Think about switching tabs or views without waiting for reload.

Describe how to control which components are cached by and mention relevant lifecycle hooks.

Focus on props and lifecycle hooks related to caching.

Practice

1. What is the main purpose of using <keep-alive> in Vue?

easy

A. To cache components and preserve their state when toggled

B. To permanently remove components from the DOM

C. To force components to reload every time they are shown

D. To style components with CSS animations

Solution

1. Step 1: Understand what <keep-alive> does

   <keep-alive> caches components so they don't reload when toggled.

2. Step 2: Compare options to this behavior

   Only 'To cache components and preserve their state when toggled' describes caching and preserving state, which is the main purpose.

3. Final Answer:

   To cache components and preserve their state when toggled -> Option A

4. Quick Check:

   Keep-alive caches components [OK]

Hint: Remember: keep-alive means keep component state alive [OK]

Common Mistakes:

• Thinking keep-alive removes components
• Confusing keep-alive with styling or animations
• Assuming keep-alive forces reloads

2. Which of the following is the correct syntax to wrap a dynamic component with keep-alive in Vue 3?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall Vue 3 syntax for keep-alive

   The <keep-alive> tag wraps the dynamic <component> with :is binding.

2. Step 2: Check each option's correctness

   <keep-alive><component :is="currentView" /></keep-alive> correctly wraps <component :is="currentView" /> inside <keep-alive>. Others misuse attributes or tags.

3. Final Answer:

   <keep-alive><component :is="currentView" /></keep-alive> -> Option A

4. Quick Check:

   Wrap dynamic component inside keep-alive tags [OK]

Hint: Wrap inside tags [OK]

Common Mistakes:

• Using keep-alive as a self-closing tag with :is
• Adding keep-alive as a prop on component
• Nesting currentView as a tag inside component

3. Given this Vue template:

<keep-alive include="CompA,CompB">
  <component :is="currentComp" />
</keep-alive>

If currentComp changes from 'CompA' to 'CompC' and back to 'CompA', what happens to the state of 'CompA'?

medium

A. State is preserved only if 'CompC' is also included

B. State is lost because 'CompA' is not included in keep-alive

C. State is preserved because 'CompA' is included in keep-alive

D. State is always reset on component switch

Solution

1. Step 1: Understand include prop effect

   The include attribute caches only components named 'CompA' and 'CompB'.

2. Step 2: Analyze component switching

   Switching to 'CompC' unloads it (not cached). Returning to 'CompA' restores cached state.

3. Final Answer:

   State is preserved because 'CompA' is included in keep-alive -> Option C

4. Quick Check:

   Include caches listed components [OK]

Hint: Only included components keep state cached [OK]

Common Mistakes:

• Assuming all components are cached regardless of include
• Thinking state resets on every switch
• Confusing include with exclude

4. Consider this Vue code snippet:

<keep-alive exclude="CompB">
  <component :is="currentComp" />
</keep-alive>

When switching from 'CompA' to 'CompB', the state of 'CompA' is lost. Why?

medium

A. Because 'CompB' is excluded, switching to it destroys cached components including 'CompA'

B. Because 'CompB' is excluded, only 'CompB' is not cached; 'CompA' should keep state

C. Because 'CompB' is excluded, keep-alive disables caching for all components

D. Because exclude disables keep-alive entirely

Solution

1. Step 1: Understand exclude behavior

   The exclude attribute prevents caching only for 'CompB'. Other components like 'CompA' remain cached.

2. Step 2: Analyze why 'CompA' state is lost

   If 'CompA' state is lost, likely the component is unmounted or recreated outside keep-alive scope, not due to exclude.

3. Final Answer:

   Because 'CompB' is excluded, only 'CompB' is not cached; 'CompA' should keep state -> Option B

4. Quick Check:

   Exclude only skips caching excluded components [OK]

Hint: Exclude skips caching only for listed components [OK]

Common Mistakes:

• Thinking exclude disables caching for all components
• Assuming exclude destroys other cached components
• Confusing exclude with disabling keep-alive

5. You have two expensive components, CompX and CompY. You want to cache CompX but not CompY. Which is the best way to use <keep-alive> to achieve this?

hard

A.

B.

C.

D.

Solution

1. Step 1: Identify caching goal

   You want to cache only CompX, not CompY.

2. Step 2: Use include to cache specific components

   Setting include="CompX" caches only CompX. CompY won't be cached.

3. Step 3: Check other options

   Exclude CompX caches everything except CompX (wrong). Include CompY caches only CompY (wrong). No filters caches all (wrong).

4. Final Answer:

   <keep-alive include="CompX"><component :is="currentComp" /></keep-alive> -> Option D

5. Quick Check:

   Include caches only listed components [OK]

Hint: Use include to cache only desired components [OK]

Common Mistakes:

• Using exclude to cache only one component
• Including the wrong component name
• Not using include or exclude and caching all