Lifecycle Hooks
All Svelte topics∙ Svelte
Lifecycle Hooks explains component callbacks for mount, update, and destroy phases for this lifecycle, hooks lesson. You will learn its exact Svelte rule, failure mode, verification plan, and production evidence.
Syntax
onMount(() => { return cleanup; });Example
// Topic: Lifecycle Hooks
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
- 1Lifecycle Hooks is used for subscriptions, timers, DOM integrations, and data refresh.
- 2Its mechanism is component callbacks for mount, update, and destroy phases for this lifecycle, hooks lesson.
- 3Attach external resources to the smallest owning lifecycle and clean them symmetrically. Keep decisions specific to lifecycle, hooks.
- 4Production code must account for Lifecycle effects without cleanup survive after the component disappears. Do not copy assumptions from a neighboring topic into lifecycle, hooks.
- 5Teams evaluate it using resource cleanup measured for lifecycle, hooks.
Common Mistakes
- 1Lifecycle effects without cleanup survive after the component disappears. Do not copy assumptions from a neighboring topic into lifecycle, hooks.
- 2Implementing Lifecycle Hooks without understanding component callbacks for mount, update, and destroy phases for this lifecycle, hooks lesson.
- 3Choosing Lifecycle Hooks where simpler local Svelte code is clearer.
- 4Skipping Mount, update, destroy, and verify timers, listeners, and subscriptions. Include an assertion that directly exercises lifecycle, hooks.
- 5Optimizing before measuring resource cleanup measured for lifecycle, hooks.
Best Practices
- 1Attach external resources to the smallest owning lifecycle and clean them symmetrically. Keep decisions specific to lifecycle, hooks.
- 2Document component callbacks for mount, update, and destroy phases for this lifecycle, hooks lesson in the smallest useful component, store, action, route, or service.
- 3Represent every relevant loading, success, empty, denied, and failure state.
- 4Mount, update, destroy, and verify timers, listeners, and subscriptions. Include an assertion that directly exercises lifecycle, hooks.
- 5Use resource cleanup measured for lifecycle, hooks to guide improvements.
How it works
- 1Lifecycle Hooks relies on component callbacks for mount, update, and destroy phases for this lifecycle, hooks lesson.
- 2Attach external resources to the smallest owning lifecycle and clean them symmetrically. Keep decisions specific to lifecycle, hooks.
- 3Its main failure mode is Lifecycle effects without cleanup survive after the component disappears. Do not copy assumptions from a neighboring topic into lifecycle, hooks.
- 4Useful evidence is resource cleanup measured for lifecycle, hooks.
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
- 1Mount, update, destroy, and verify timers, listeners, and subscriptions. Include an assertion that directly exercises lifecycle, hooks.
- 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 Lifecycle Hooks example.
- 2Introduce this failure: Lifecycle effects without cleanup survive after the component disappears. Do not copy assumptions from a neighboring topic into lifecycle, hooks.
- 3Correct it using this rule: Attach external resources to the smallest owning lifecycle and clean them symmetrically. Keep decisions specific to lifecycle, hooks.
- 4Record resource cleanup measured for lifecycle, hooks before and after the change.
Quick Summary
- Lifecycle Hooks works through component callbacks for mount, update, and destroy phases for this lifecycle, hooks lesson.
- Attach external resources to the smallest owning lifecycle and clean them symmetrically. Keep decisions specific to lifecycle, hooks.
- Avoid Lifecycle effects without cleanup survive after the component disappears. Do not copy assumptions from a neighboring topic into lifecycle, hooks.
- Mount, update, destroy, and verify timers, listeners, and subscriptions. Include an assertion that directly exercises lifecycle, hooks.
- Measure success with resource cleanup measured for lifecycle, hooks.
Interview Questions
Q1. What is Lifecycle Hooks used for?
Answer: It is used for subscriptions, timers, DOM integrations, and data refresh.
Q2. How does Lifecycle Hooks work in Svelte?
Answer: It works through component callbacks for mount, update, and destroy phases for this lifecycle, hooks lesson.
Q3. What rule matters most?
Answer: Attach external resources to the smallest owning lifecycle and clean them symmetrically. Keep decisions specific to lifecycle, hooks.
Q4. What failure is common?
Answer: Lifecycle effects without cleanup survive after the component disappears. Do not copy assumptions from a neighboring topic into lifecycle, hooks.
Q5. How should it be verified?
Answer: Mount, update, destroy, and verify timers, listeners, and subscriptions. Include an assertion that directly exercises lifecycle, hooks. Evaluate resource cleanup measured for lifecycle, hooks.
Quiz
Which practice best supports Lifecycle Hooks?