onDestroy Hook
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onDestroy Hook explains the final component cleanup callback for this ondestroy, hook lesson. You will learn its exact Svelte rule, failure mode, verification plan, and production evidence.
Syntax
onMount(() => { return cleanup; });Example
// Topic: onDestroy Hook
const lifecycle = ['mount', 'update', 'destroy'];
console.log(lifecycle.join(' -> '));
// Expected Output: mount -> update -> destroyExpected Output
mount -> update -> destroyLine-by-line
| Line | Meaning |
|---|---|
const lifecycle = ['mount', 'update', 'destroy']; | Defines state, behavior, or output for this Svelte example. |
console.log(lifecycle.join(' -> ')); | Prints the expected result for this Svelte lesson. |
Real-World Uses
- 1onDestroy Hook is used for subscriptions, timers, DOM integrations, and data refresh.
- 2Its mechanism is the final component cleanup callback for this ondestroy, hook lesson.
- 3Release listeners, timers, subscriptions, and pending work symmetrically. Keep decisions specific to ondestroy, hook.
- 4Production code must account for Assuming garbage collection removes external subscriptions leaks behavior. Do not copy assumptions from a neighboring topic into ondestroy, hook.
- 5Teams evaluate it using zero leaked resources measured for ondestroy, hook.
Common Mistakes
- 1Assuming garbage collection removes external subscriptions leaks behavior. Do not copy assumptions from a neighboring topic into ondestroy, hook.
- 2Implementing onDestroy Hook without understanding the final component cleanup callback for this ondestroy, hook lesson.
- 3Choosing onDestroy Hook where simpler local Svelte code is clearer.
- 4Skipping Unmount and verify every owned resource stops. Include an assertion that directly exercises ondestroy, hook.
- 5Optimizing before measuring zero leaked resources measured for ondestroy, hook.
Best Practices
- 1Release listeners, timers, subscriptions, and pending work symmetrically. Keep decisions specific to ondestroy, hook.
- 2Document the final component cleanup callback for this ondestroy, hook lesson in the smallest useful component, store, action, route, or service.
- 3Represent every relevant loading, success, empty, denied, and failure state.
- 4Unmount and verify every owned resource stops. Include an assertion that directly exercises ondestroy, hook.
- 5Use zero leaked resources measured for ondestroy, hook to guide improvements.
How it works
- 1onDestroy Hook relies on the final component cleanup callback for this ondestroy, hook lesson.
- 2Release listeners, timers, subscriptions, and pending work symmetrically. Keep decisions specific to ondestroy, hook.
- 3Its main failure mode is Assuming garbage collection removes external subscriptions leaks behavior. Do not copy assumptions from a neighboring topic into ondestroy, hook.
- 4Useful evidence is zero leaked resources measured for ondestroy, hook.
Implementation decisions
- 1Identify the owning component, store, action, route, load function, or server handler.
- 2Keep state local until multiple owners genuinely need it.
- 3Keep server secrets and validation outside browser components.
- 4Define cleanup for subscriptions, actions, timers, and requests.
Verification plan
- 1Unmount and verify every owned resource stops. Include an assertion that directly exercises ondestroy, hook.
- 2Check initial render, assignment-driven updates, user interaction, and cleanup.
- 3Confirm keyboard and screen-reader behavior for visible UI.
- 4Measure production output only after correctness passes.
Practice task
- 1Build the smallest onDestroy Hook example.
- 2Introduce this failure: Assuming garbage collection removes external subscriptions leaks behavior. Do not copy assumptions from a neighboring topic into ondestroy, hook.
- 3Correct it using this rule: Release listeners, timers, subscriptions, and pending work symmetrically. Keep decisions specific to ondestroy, hook.
- 4Record zero leaked resources measured for ondestroy, hook before and after the change.
Quick Summary
- onDestroy Hook works through the final component cleanup callback for this ondestroy, hook lesson.
- Release listeners, timers, subscriptions, and pending work symmetrically. Keep decisions specific to ondestroy, hook.
- Avoid Assuming garbage collection removes external subscriptions leaks behavior. Do not copy assumptions from a neighboring topic into ondestroy, hook.
- Unmount and verify every owned resource stops. Include an assertion that directly exercises ondestroy, hook.
- Measure success with zero leaked resources measured for ondestroy, hook.
Interview Questions
Q1. What is onDestroy Hook used for?
Answer: It is used for subscriptions, timers, DOM integrations, and data refresh.
Q2. How does onDestroy Hook work in Svelte?
Answer: It works through the final component cleanup callback for this ondestroy, hook lesson.
Q3. What rule matters most?
Answer: Release listeners, timers, subscriptions, and pending work symmetrically. Keep decisions specific to ondestroy, hook.
Q4. What failure is common?
Answer: Assuming garbage collection removes external subscriptions leaks behavior. Do not copy assumptions from a neighboring topic into ondestroy, hook.
Q5. How should it be verified?
Answer: Unmount and verify every owned resource stops. Include an assertion that directly exercises ondestroy, hook. Evaluate zero leaked resources measured for ondestroy, hook.
Quiz
Which practice best supports onDestroy Hook?