onDestroy in Svelte

Introduction

The onDestroy function lets you run code when a Svelte component is removed from the page. This helps clean up things like timers or event listeners so your app stays fast and tidy.

You start a timer or interval in a component and want to stop it when the component is gone.

You add event listeners to the window or document and want to remove them when the component is removed.

You open a WebSocket or other connection and want to close it when the component is destroyed.

You want to clear any resources or cancel subscriptions when the component is no longer visible.

Syntax

Svelte

import { onDestroy } from 'svelte';

onDestroy(() => {
  // cleanup code here
});

You import onDestroy from 'svelte' to use it.

The function you pass to onDestroy runs once when the component is removed.

Examples

This logs a message when the component is destroyed.

Svelte

import { onDestroy } from 'svelte';

onDestroy(() => {
  console.log('Component is gone');
});

This example starts a timer and stops it when the component is removed.

Svelte

import { onDestroy } from 'svelte';

const interval = setInterval(() => {
  console.log('Tick');
}, 1000);

onDestroy(() => {
  clearInterval(interval);
});

Sample Program

This component increases a number every second. When the component is removed, the timer stops and a message logs in the console.

Svelte

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

  let count = 0;

  const interval = setInterval(() => {
    count += 1;
  }, 1000);

  onDestroy(() => {
    clearInterval(interval);
    console.log('Interval cleared');
  });
</script>

<p>Count: {count}</p>

Important Notes

Always clean up timers, listeners, or subscriptions in onDestroy to avoid memory leaks.

onDestroy runs only once per component instance.

Summary

onDestroy runs code when a Svelte component is removed.

Use it to clean up timers, event listeners, or other resources.

Helps keep your app fast and bug-free by avoiding leftover tasks.

Practice

1. What is the main purpose of the onDestroy function in a Svelte component?

easy

A. To run cleanup code when the component is removed from the page

B. To initialize component state when it loads

C. To update the component when data changes

D. To handle user input events

Solution

1. Step 1: Understand the lifecycle of a Svelte component

   The onDestroy function runs when the component is about to be removed from the page.

2. Step 2: Identify the purpose of onDestroy

   It is used to clean up things like timers or event listeners to avoid bugs or memory leaks.

3. Final Answer:

   To run cleanup code when the component is removed from the page -> Option A

4. Quick Check:

   onDestroy = cleanup on removal [OK]

Hint: Remember: onDestroy cleans up when component leaves [OK]

Common Mistakes:

• Confusing onDestroy with onMount
• Thinking onDestroy runs on data update
• Using onDestroy to initialize state

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

easy

A. import { onDestroy } from 'svelte'; onDestroy.cleanup(() => { /* cleanup */ });

B. import onDestroy from 'svelte'; onDestroy(() => { /* cleanup */ });

C. import { onDestroy } from 'svelte'; onDestroy(() => { /* cleanup */ });

D. import { onDestroy } from 'svelte'; onDestroy = () => { /* cleanup */ };

Solution

1. Step 1: Check the import syntax

   The correct import is a named import: import { onDestroy } from 'svelte';

2. Step 2: Check how to use onDestroy

   You call onDestroy with a function to run cleanup code: onDestroy(() => { ... });

3. Final Answer:

   import { onDestroy } from 'svelte'; onDestroy(() => { /* cleanup */ }); -> Option C

4. Quick Check:

   Correct import and call syntax = import { onDestroy } from 'svelte'; onDestroy(() => { /* cleanup */ }); [OK]

Hint: Use named import and call onDestroy with a function [OK]

Common Mistakes:

• Using default import instead of named import
• Assigning onDestroy instead of calling it
• Calling a non-existent cleanup method

3. Consider this Svelte component code:

import { onDestroy } from 'svelte';

let timer;
onDestroy(() => {
  clearInterval(timer);
});

timer = setInterval(() => {
  console.log('Tick');
}, 1000);

What happens when this component is removed from the page?

medium

A. The interval timer continues running and logs 'Tick' forever

B. The component reloads automatically

C. An error occurs because clearInterval is called incorrectly

D. The interval timer stops and no more 'Tick' logs appear

Solution

1. Step 1: Understand the timer setup

   The code sets a repeating timer with setInterval that logs 'Tick' every second.

2. Step 2: Analyze the onDestroy cleanup

   The onDestroy callback calls clearInterval(timer), which stops the timer when the component is removed.

3. Final Answer:

   The interval timer stops and no more 'Tick' logs appear -> Option D

4. Quick Check:

   onDestroy clears timer = stops logs [OK]

Hint: onDestroy stops timers to prevent leftover logs [OK]

Common Mistakes:

• Thinking timer keeps running after component removal
• Confusing clearInterval with setTimeout
• Assuming onDestroy runs multiple times

4. This Svelte component tries to clean up an event listener on destroy:

import { onDestroy } from 'svelte';

function handleClick() {
  console.log('Clicked');
}

window.addEventListener('click', handleClick);

onDestroy(() => {
  window.removeEventListener('click');
});

What is wrong with this code?

medium

A. removeEventListener is missing the event handler function argument

B. onDestroy should be called before addEventListener

C. handleClick should be defined inside onDestroy

D. window.addEventListener should be inside onDestroy

Solution

1. Step 1: Check removeEventListener usage

   To remove an event listener, you must pass both the event type and the exact handler function.

2. Step 2: Identify the missing argument

   The code calls window.removeEventListener('click'); without the handler handleClick, so the listener is not removed.

3. Final Answer:

   removeEventListener is missing the event handler function argument -> Option A

4. Quick Check:

   removeEventListener needs event and handler [OK]

Hint: Always pass event and handler to removeEventListener [OK]

Common Mistakes:

• Omitting the handler function in removeEventListener
• Adding listeners inside onDestroy instead of removing
• Defining handler inside onDestroy causing scope issues

5. You want to use onDestroy to clean up multiple resources: a timer and a custom event listener. Which is the best way to do this in a Svelte component?

import { onDestroy } from 'svelte';

let timer = setInterval(() => console.log('Tick'), 1000);

function onCustomEvent() { console.log('Event'); }

window.addEventListener('custom', onCustomEvent);

// What is the correct onDestroy usage?

hard

A. onDestroy(() => { clearInterval(timer); window.removeEventListener('custom'); });

B. onDestroy(() => { clearInterval(timer); window.removeEventListener('custom', onCustomEvent); });

C. onDestroy(() => { clearInterval(timer); }); onDestroy(() => { window.removeEventListener('custom'); });

D. onDestroy(() => { clearInterval(); window.removeEventListener('custom', onCustomEvent); });

Solution

1. Step 1: Combine cleanup tasks in one onDestroy callback

   onDestroy accepts a single function where you can clean multiple resources.

2. Step 2: Correctly clear timer and remove event listener

   Use clearInterval(timer) and window.removeEventListener('custom', onCustomEvent) with proper arguments.

3. Final Answer:

   onDestroy(() => { clearInterval(timer); window.removeEventListener('custom', onCustomEvent); }); -> Option B

4. Quick Check:

   Single onDestroy with all cleanup calls [OK]

Hint: Put all cleanup code inside one onDestroy callback [OK]

Common Mistakes:

• Using multiple onDestroy calls instead of one
• Omitting handler in removeEventListener
• Calling clearInterval without timer argument