Lifecycle in nested components in Svelte - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to run a function when the component is first added to the page.

Svelte

<script>
  import { [1] } from 'svelte';

  onMount(() => {
    console.log('Component mounted');
  });
</script>

Drag options to blanks, or click blank then click option'

AonMount

BonDestroy

CbeforeUpdate

DafterUpdate

Attempts:

3 left

💡 Hint

Common Mistakes

Using onDestroy instead of onMount

Confusing onMount with beforeUpdate

The onMount function runs code when the component is first added to the page.

2fill in blank

medium

Complete the code to clean up when a nested component is removed.

Svelte

<script>
  import { onDestroy } from 'svelte';

  onDestroy(() => {
    [1];
  });
</script>

Drag options to blanks, or click blank then click option'

Aconsole.log('Component created')

Bconsole.log('Component mounted')

Cconsole.log('Component updated')

Dconsole.log('Component removed')

Attempts:

3 left

💡 Hint

Common Mistakes

Logging 'Component mounted' inside onDestroy

Using onMount code inside onDestroy

The onDestroy function runs cleanup code when the component is removed from the page.

3fill in blank

hard

Fix the error in the nested component lifecycle hook usage.

Svelte

<script>
  import { onMount } from 'svelte';

  onMount(() => {
    console.log('Nested component [1]');
  });
</script>

Drag options to blanks, or click blank then click option'

Aupdated

Bdestroyed

Cmounted

Dcreated

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'updated' or 'destroyed' in onMount message

Confusing lifecycle stages

The onMount hook runs when the component is mounted, so the message should say 'mounted'.

4fill in blank

hard

Fill both blanks to run code before and after the component updates.

Svelte

<script>
  import { [1], [2] } from 'svelte';

  [1](() => {
    console.log('Before update');
  });

  [2](() => {
    console.log('After update');
  });
</script>

Drag options to blanks, or click blank then click option'

AbeforeUpdate

BonMount

CafterUpdate

DonDestroy

Attempts:

3 left

💡 Hint

Common Mistakes

Using onMount or onDestroy instead of update hooks

Mixing up beforeUpdate and afterUpdate

beforeUpdate runs code before the component updates, and afterUpdate runs code after the update.

5fill in blank

hard

Fill all three blanks to correctly handle lifecycle in a nested component.

Svelte

<script>
  import { [1], [2], [3] } from 'svelte';

  [1](() => {
    console.log('Mounted');
  });

  [2](() => {
    console.log('Before update');
  });

  [3](() => {
    console.log('Cleanup on destroy');
  });
</script>

Drag options to blanks, or click blank then click option'

AonMount

BbeforeUpdate

ConDestroy

DafterUpdate

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up onDestroy and afterUpdate

Using afterUpdate instead of beforeUpdate for 'Before update' message

onMount runs when the component appears, beforeUpdate runs before changes, and onDestroy runs cleanup when removed.

Practice

1. In Svelte, when you have nested components, which lifecycle method runs first in the child component after it is added to the DOM?

easy

A. onMount

B. onDestroy

C. beforeUpdate

D. afterUpdate

Solution

1. Step 1: Understand lifecycle order in nested components

   When a child component is added to the DOM, its onMount runs after the component is rendered.

2. Step 2: Identify the first lifecycle method after mounting

   onMount is the first lifecycle method called after the component is inserted, before any updates or destruction.

3. Final Answer:

   onMount -> Option A

4. Quick Check:

   Child component mounts first with onMount [OK]

Hint: onMount runs first when a component appears [OK]

Common Mistakes:

• Confusing onDestroy as first method
• Thinking beforeUpdate runs before onMount
• Assuming afterUpdate runs on initial mount

2. Which of the following is the correct way to import and use onMount in a nested Svelte component?

easy

A. import { onMount } from 'svelte/lifecycle'; onMount(() => { console.log('Mounted'); });

B. import onMount from 'svelte'; onMount(() => { console.log('Mounted'); });

C. import { onMount } from 'svelte'; onMount(() => { console.log('Mounted'); });

D. import { onMount } from 'svelte'; onMount = () => { console.log('Mounted'); };

Solution

1. Step 1: Check correct import syntax

   The correct import is import { onMount } from 'svelte'; using named import from 'svelte'.

2. Step 2: Verify usage of onMount function

   Calling onMount(() => { ... }) registers the callback properly. Assigning onMount to a function is incorrect.

3. Final Answer:

   import { onMount } from 'svelte'; onMount(() => { console.log('Mounted'); }); -> Option C

4. Quick Check:

   Named import and callback usage [OK]

Hint: Use named import from 'svelte' for lifecycle hooks [OK]

Common Mistakes:

• Using default import instead of named import
• Importing from wrong path like 'svelte/lifecycle'
• Assigning onMount instead of calling it

3. Consider a parent component that includes a child component. Both have onMount hooks that log messages. What is the order of console logs when the parent is rendered?

medium

A. Child's onMount logs first, then parent's onMount

B. Parent's onMount logs first, then child's onMount

C. Both onMount logs happen simultaneously

D. No logs appear because onMount doesn't run in nested components

Solution

1. Step 1: Understand mounting order in nested components

   Child components mount before their parents finish mounting, so child's onMount runs first.

2. Step 2: Confirm console log order

   Child's onMount logs first, then parent's onMount logs after child is fully mounted.

3. Final Answer:

   Child's onMount logs first, then parent's onMount -> Option A

4. Quick Check:

   Child mounts before parent finishes [OK]

Hint: Child onMount runs before parent onMount [OK]

Common Mistakes:

• Assuming parent mounts before child
• Thinking onMount runs simultaneously
• Believing onMount doesn't run in nested components

4. You have a nested component with an onDestroy hook, but it never runs when the component is removed. What is the most likely cause?

medium

A. The parent component's onDestroy prevents child's onDestroy

B. The component is never actually removed from the DOM

C. You used onMount instead of onDestroy

D. You forgot to import onDestroy from 'svelte'

Solution

1. Step 1: Check if component is removed from DOM

   onDestroy only runs when the component is removed. If it stays in DOM, it won't run.

2. Step 2: Verify other causes

   Importing onDestroy incorrectly or mixing hooks causes errors, but the main reason is component not unmounting.

3. Final Answer:

   The component is never actually removed from the DOM -> Option B

4. Quick Check:

   onDestroy runs only on removal [OK]

Hint: onDestroy runs only when component leaves DOM [OK]

Common Mistakes:

• Expecting onDestroy without unmounting
• Confusing onMount with onDestroy
• Assuming parent's lifecycle affects child's onDestroy

5. You have a nested component that fetches data in onMount and cleans up a subscription in onDestroy. If the parent conditionally renders this child multiple times, what is the best practice to avoid memory leaks?

hard

A. Avoid conditional rendering of the child component

B. Move the data fetching and subscription to the parent component only

C. Use beforeUpdate in the child to reset subscriptions

D. Ensure the child's onDestroy properly unsubscribes every time it unmounts

Solution

1. Step 1: Understand lifecycle with conditional rendering

   When the child mounts and unmounts repeatedly, onDestroy must clean up resources each time to prevent leaks.

2. Step 2: Evaluate best practice for cleanup

   Properly unsubscribing in onDestroy ensures no leftover subscriptions remain after unmount.

3. Final Answer:

   Ensure the child's onDestroy properly unsubscribes every time it unmounts -> Option D

4. Quick Check:

   Clean up in onDestroy to avoid leaks [OK]

Hint: Always clean subscriptions in onDestroy [OK]

Common Mistakes:

• Ignoring cleanup in onDestroy
• Moving logic unnecessarily to parent
• Using beforeUpdate for cleanup instead of onDestroy