Lifecycle in nested components in Svelte - Mini Project: Build & Apply

Lifecycle in nested components

📖 Scenario: You are building a simple Svelte app with a parent component and a nested child component. You want to see how lifecycle functions work in both components.

🎯 Goal: Create a parent component and a nested child component. Use Svelte lifecycle functions onMount and onDestroy in both components to log messages when they mount and unmount.

📋 What You'll Learn

Create a Svelte component called Child.svelte with onMount and onDestroy lifecycle functions that log messages.

Create a Svelte component called Parent.svelte that imports Child.svelte and uses it inside its markup.

Add onMount and onDestroy lifecycle functions in Parent.svelte that log messages.

Render Child inside Parent so you can observe lifecycle logs for both.

💡 Why This Matters

🌍 Real World

Understanding lifecycle in nested components helps you build interactive apps where components mount and unmount dynamically, such as tabs, modals, or lists.

💼 Career

Many frontend jobs require knowledge of component lifecycles to manage resources, fetch data, and clean up properly in frameworks like Svelte.

1

Create the Child component with lifecycle logs

Create a Svelte component file named Child.svelte. Inside it, import onMount and onDestroy from svelte. Use onMount to log the message "Child mounted" and onDestroy to log "Child destroyed". Add a simple <p> tag with the text "I am the child component".

Svelte

  // Import lifecycle functions and add onMount and onDestroy
  // Your code here


I am the child component

Hint

Remember to import onMount and onDestroy from svelte and use them as functions inside the <script> tag.

2

Create the Parent component with lifecycle logs

Create a Svelte component file named Parent.svelte. Import onMount and onDestroy from svelte. Use onMount to log the message "Parent mounted" and onDestroy to log "Parent destroyed". Add a simple <h1> tag with the text "I am the parent component".

Svelte

  // Import lifecycle functions and add onMount and onDestroy
  // Your code here


I am the parent component

Hint

Use the same lifecycle functions as in the child, but with messages for the parent.

3

Import and use Child component inside Parent

In Parent.svelte, import the Child component from ./Child.svelte. Add the <Child /> tag below the <h1> tag to render the child component inside the parent.

Svelte

  import { onMount, onDestroy } from 'svelte';
  // Import Child component here
  // Your code here

  onMount(() => {
    console.log("Parent mounted");
  });

  onDestroy(() => {
    console.log("Parent destroyed");
  });


I am the parent component

Hint

Use import Child from './Child.svelte' and add <Child /> in the markup.

4

Complete the app by exporting Parent as default

In Parent.svelte, add export default for the component by ensuring the file is saved as a Svelte component. No extra code is needed, just confirm the file exports the component by default.

Svelte

  import { onMount, onDestroy } from 'svelte';
  import Child from './Child.svelte';

  onMount(() => {
    console.log("Parent mounted");
  });

  onDestroy(() => {
    console.log("Parent destroyed");
  });


I am the parent component

Hint

Just save the file as Parent.svelte. Svelte automatically exports the component.

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