Lifecycle Methods Interview Questions

Lifecycle methods in Svelte are functions that execute at different stages of a component's existence. Main lifecycle methods include onMount, onDestroy, beforeUpdate, and afterUpdate. They help manage side effects and component behavior.

onMount is a lifecycle function that runs after the component is first rendered to the DOM. It's commonly used for initialization, data fetching, and setting up subscriptions. Returns a cleanup function optionally.

onDestroy is called when a component is unmounted from the DOM. Used for cleanup like unsubscribing from stores, clearing intervals, or removing event listeners. Prevents memory leaks.

beforeUpdate runs before the DOM is updated with new values. Useful for capturing pre-update state, like scroll position. Can be used multiple times in a component.

afterUpdate runs after the DOM is updated with new values. Used for operations that require updated DOM state, like updating scroll position or third-party libraries.

Async operations in onMount use async/await or promises. Handle loading states and errors appropriately. Example: onMount(async () => { const data = await fetchData(); })

Lifecycle methods execute in order: 1. Component creation, 2. beforeUpdate, 3. onMount, 4. afterUpdate. onDestroy runs when component is unmounted. Updates trigger beforeUpdate and afterUpdate.

Resource cleanup is done in onDestroy or by returning cleanup function from onMount. Clear intervals, timeouts, event listeners, and subscriptions to prevent memory leaks.

Svelte guarantees that onMount runs after DOM is ready, onDestroy runs before unmount, beforeUpdate before DOM updates, and afterUpdate after DOM updates. Component initialization always runs.

During SSR, onMount and afterUpdate don't run. Other lifecycle methods run normally. Client-side hydration triggers lifecycle methods appropriately. Handle SSR-specific logic.

Error handling uses try/catch blocks or error boundaries. Handle async errors appropriately. Implement error recovery strategies. Support error reporting.

State initialization happens in component creation or onMount. Handle async initialization. Support loading states. Implement initialization strategies.

Store subscriptions are typically set up in onMount and cleaned in onDestroy. Handle store updates. Support auto-subscription. Manage subscription lifecycle.

DOM measurements use afterUpdate for accurate values. Cache measurements when needed. Handle resize events. Support responsive measurements.

Initialization patterns include lazy loading, conditional initialization, and dependency injection. Handle initialization order. Support async initialization.

Prop changes trigger beforeUpdate and afterUpdate. Implement prop watchers. Handle derived values. Support prop validation in lifecycle.

Side effects management includes cleanup, ordering, and dependencies. Handle async side effects. Support cancellation. Implement side effect tracking.

Animations interact with beforeUpdate and afterUpdate. Handle transition states. Support animation cleanup. Implement animation queuing.

Lifecycle hooks extend default lifecycle behavior. Implement custom hooks. Support hook composition. Handle hook dependencies.

Component transitions use lifecycle methods for coordination. Handle mount/unmount animations. Support transition states. Implement transition hooks.

Advanced initialization includes dependency graphs, async loading, and state machines. Handle complex dependencies. Support initialization rollback.

Lifecycle monitoring tracks method execution and performance. Implement monitoring tools. Handle performance tracking. Support debugging.

Lifecycle testing verifies method execution and timing. Implement test utilities. Handle async testing. Support integration testing.

Lifecycle optimization improves performance and efficiency. Handle method batching. Implement update optimization. Support selective updates.

Lifecycle documentation describes method behavior and usage. Generate documentation automatically. Support example usage. Handle versioning.

Complex cleanup handles nested resources and dependencies. Implement cleanup ordering. Support partial cleanup. Handle cleanup failures.

Error boundaries catch and handle lifecycle errors. Implement recovery strategies. Support fallback content. Handle error reporting.

State machines manage complex lifecycle states. Handle state transitions. Support parallel states. Implement state persistence.

Lifecycle debugging tracks method execution and state. Implement debugging tools. Support breakpoints. Handle debugging output.

Lifecycle security prevents unauthorized access and manipulation. Handle sensitive operations. Support access control. Implement security policies.