Svelte Interview Questions Your Guide to Success

Svelte's unique compile-time approach and reactive programming model make it a powerful framework for modern web development. Stark.ai offers a comprehensive collection of Svelte interview questions, real-world scenarios, and expert guidance to help you excel in your next technical interview.

Svelte Basics & Components 10
Reactivity & State Management 10
Props & Component Communication 10
Lifecycle Methods 10
Events & Event Handling 10
Stores & Global State 10
Transitions & Animations 10
Slots & Content Projection 19
Routing in Svelte (SvelteKit) 10
Context API & Dependencies 10
Bindings & Directives 10
Server Side Rendering (SSR) 10
Testing & Debugging 10
Performance & Optimization 10

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Svelte Basics & Components
- What is Svelte and how is it different from other frameworks?
  
  Svelte is a compiler that creates reactive JavaScript modules. Unlike React or Vue that do most of their work in the...
- How do you create a basic Svelte component?
  
  A Svelte component is created in a .svelte file with three main sections: <script> for JavaScript logic, <style> for...
- How do you declare reactive variables in Svelte?
  
  Reactive variables in Svelte are declared using the let keyword in the <script> section. Any variables used in the...
- What is the purpose of the $ syntax in Svelte?
  
  The $ prefix in Svelte is a special syntax that marks a statement as reactive. It automatically re-runs the code...
- How do you include conditional rendering in Svelte?
  
  Svelte uses #if, :else if, and :else blocks for conditional rendering. Example: {#if condition}...{:else if...
- How do you create loops in Svelte templates?
  
  Svelte uses the #each block for iteration. Syntax: {#each items as item, index}...{/each}. Supports destructuring,...
- What is the purpose of export keyword in Svelte?
  
  The export keyword in Svelte is used to declare props that a component can receive. Example: export let name; Makes...
- How do you handle basic events in Svelte?
  
  Events in Svelte are handled using the on: directive. Example: <button on:click={handleClick}>. Supports modifiers...
- What is component composition in Svelte?
  
  Component composition in Svelte involves building larger components from smaller ones. Components can be imported...
- How do you add styles to Svelte components?
  
  Styles are added in the <style> block and are automatically scoped to the component. Global styles can be added...
Reactivity & State Management
- What is reactivity in Svelte?
  
  Reactivity in Svelte is the automatic updating of the DOM when data changes. It's handled through assignments to...
- How do you create a writable store in Svelte?
  
  Writable stores are created using writable() from svelte/store. Example: const count = writable(0). Provides methods...
- What are reactive declarations in Svelte?
  
  Reactive declarations use the $: syntax to automatically recompute values when dependencies change. Example: $:...
- How do you update arrays reactively in Svelte?
  
  Arrays must be updated using assignment for reactivity. Use methods like [...array, newItem] for additions,...
- What is the difference between writable and readable stores?
  
  Writable stores can be modified using set() and update(), while readable stores are read-only. Readable stores are...
- How do you subscribe to stores in Svelte?
  
  Stores can be subscribed to using subscribe() method or automatically in templates using $ prefix. Example: $store...
- What is auto-subscription in Svelte?
  
  Auto-subscription happens when using $ prefix with stores in components. Svelte automatically handles subscribe and...
- How do you handle derived values in Svelte?
  
  Derived values can be created using reactive declarations ($:) or derived stores. They automatically update when...
- What is the purpose of the set function in stores?
  
  The set function directly sets a new value for a writable store. Example: count.set(10). Triggers store updates and...
- How do you update objects reactively in Svelte?
  
  Objects must be updated through assignment for reactivity. Use spread operator or Object.assign for updates....
Props & Component Communication
- How do you declare props in Svelte components?
  
  Props are declared using the export keyword in the script section. Example: export let propName. Optional props can...
- How do you pass props to child components?
  
  Props are passed to child components as attributes in the markup. Example: <ChildComponent propName={value} />. Can...
- What are spread props in Svelte?
  
  Spread props allow passing multiple props using spread syntax. Example: <Component {...props} />. Useful when...
- How do you handle prop validation in Svelte?
  
  Prop validation can be handled through TypeScript types or runtime checks. Use if statements or assertions in...
- How do components communicate events to parents?
  
  Components dispatch custom events using createEventDispatcher. Parent listens using on:eventName. Example:...
- What is event forwarding in Svelte?
  
  Event forwarding passes events up through components using on:eventName. Component can forward DOM events or custom...
- How do you bind to component props?
  
  Two-way binding on props uses bind:propName. Example: <Input bind:value={name} />. Component must export the prop....
- What are reactive statements in component communication?
  
  Reactive statements ($:) respond to prop changes. Can trigger side effects or derive values. Example: $:...
- How do you pass HTML attributes through to elements?
  
  HTML attributes can be forwarded using $$props or $$restProps. Useful for wrapper components. Example: <div...
- What is prop destructuring in Svelte?
  
  Props can be destructured in export statement. Example: export let { name, age } = data. Supports default values and...
Lifecycle Methods
- What are lifecycle methods in Svelte?
  
  Lifecycle methods in Svelte are functions that execute at different stages of a component's existence. Main...
- What is onMount in Svelte?
  
  onMount is a lifecycle function that runs after the component is first rendered to the DOM. It's commonly used for...
- What is onDestroy in Svelte?
  
  onDestroy is called when a component is unmounted from the DOM. Used for cleanup like unsubscribing from stores,...
- What is beforeUpdate in Svelte?
  
  beforeUpdate runs before the DOM is updated with new values. Useful for capturing pre-update state, like scroll...
- What is afterUpdate in Svelte?
  
  afterUpdate runs after the DOM is updated with new values. Used for operations that require updated DOM state, like...
- How do you handle async operations in onMount?
  
  Async operations in onMount use async/await or promises. Handle loading states and errors appropriately. Example:...
- What is the execution order of lifecycle methods?
  
  Lifecycle methods execute in order: 1. Component creation, 2. beforeUpdate, 3. onMount, 4. afterUpdate. onDestroy...
- How do you clean up resources in lifecycle methods?
  
  Resource cleanup is done in onDestroy or by returning cleanup function from onMount. Clear intervals, timeouts,...
- What are lifecycle guarantees in Svelte?
  
  Svelte guarantees that onMount runs after DOM is ready, onDestroy runs before unmount, beforeUpdate before DOM...
- How do lifecycle methods work with SSR?
  
  During SSR, onMount and afterUpdate don't run. Other lifecycle methods run normally. Client-side hydration triggers...
Events & Event Handling
- How do you handle DOM events in Svelte?
  
  DOM events in Svelte are handled using the on: directive. Example: <button on:click={handleClick}>. Supports all...
- What are event modifiers in Svelte?
  
  Event modifiers customize event behavior using | symbol. Common modifiers include preventDefault, stopPropagation,...
- How do you create custom events in Svelte?
  
  Custom events are created using createEventDispatcher. Import from svelte, initialize in component, then dispatch...
- How do you listen to component events?
  
  Component events are listened to using on:eventName. Parent components can listen to dispatched events. Example:...
- What is event forwarding in Svelte?
  
  Event forwarding passes events up through components using on:eventName. No handler needed for forwarding. Example:...
- How do you handle inline event handlers?
  
  Inline handlers can be defined directly in the on: directive. Example: <button on:click={() => count += 1}>. Good...
- What is event delegation in Svelte?
  
  Event delegation handles events at a higher level using bubbling. Attach single handler to parent for multiple...
- How do you pass data with custom events?
  
  Data is passed as second argument to dispatch. Example: dispatch('custom', { detail: value }). Accessed in handler...
- What is the event object in Svelte?
  
  Event object contains event details passed to handlers. Includes properties like target, currentTarget,...
- How do you handle multiple events on one element?
  
  Multiple events use multiple on: directives. Example: <div on:click={handleClick} on:mouseover={handleHover}>. Each...
Stores & Global State
- What are Svelte stores?
  
  Stores in Svelte are reactive data containers that can be shared across components. They provide a way to manage...
- How do you create a writable store?
  
  Writable stores are created using writable() from svelte/store. Example: const count = writable(0). They provide...
- What is the store contract in Svelte?
  
  The store contract requires an object with subscribe method that takes a callback and returns unsubscribe function....
- How do you subscribe to store changes?
  
  Store changes can be subscribed to using subscribe method or $ prefix in components. Example: count.subscribe(value...
- What are readable stores?
  
  Readable stores are created using readable() and are read-only. They can only be updated through their start...
- How do you use derived stores?
  
  Derived stores are created using derived() and compute values based on other stores. Example: const doubled =...
- What is auto-subscription in Svelte?
  
  Auto-subscription happens when using $ prefix with stores in components. Svelte automatically handles subscribe and...
- How do you update store values?
  
  Store values can be updated using set() or update() methods. Example: count.set(10) or count.update(n => n + 1)....
- What is store initialization?
  
  Stores are initialized with initial value in creation. Example: writable(initialValue). Can be undefined. Support...
- How do stores work with reactivity?
  
  Stores integrate with Svelte's reactivity system. Changes trigger reactive updates. Work with reactive declarations...
Transitions & Animations
- What are transitions in Svelte?
  
  Transitions in Svelte are built-in animations that play when elements are added to or removed from the DOM. They are...
- How do you use the fade transition?
  
  Fade transition is used with transition:fade directive. Example: <div transition:fade>. Can accept parameters like...
- What is the difference between in: and out: directives?
  
  in: and out: directives specify different transitions for entering and leaving. Example: <div in:fade out:fly>....
- What are transition parameters?
  
  Transition parameters customize animation behavior. Example: transition:fade={{duration: 300, delay: 100}}. Include...
- How do you use the fly transition?
  
  Fly transition animates position and opacity. Example: <div transition:fly={{y: 200}}>. Parameters include x, y...
- What is the slide transition?
  
  Slide transition animates element height. Example: <div transition:slide>. Useful for collapsible content. Can...
- How do you use easing functions?
  
  Easing functions define animation progression. Import from svelte/easing. Example: transition:fade={{easing:...
- What is transition:local modifier?
  
  transition:local restricts transitions to when parent component is added/removed. Prevents transitions during...
- How do you use the scale transition?
  
  Scale transition animates size and opacity. Example: <div transition:scale>. Parameters include start, opacity,...
- What is the draw transition?
  
  Draw transition animates SVG paths. Example: <path transition:draw>. Parameters include duration, delay, easing....
Slots & Content Projection
- What are slots in Svelte?
  
  Slots are placeholders in components that allow parent components to pass content. Defined using <slot> element....
- How do you define a default slot?
  
  Default slots provide fallback content. Example: <slot>Default content</slot>. Content appears when parent doesn't...
- What are named slots?
  
  Named slots target specific slot locations using name attribute. Example: <slot name='header'>. Content provided...
- How do you pass content to named slots?
  
  Content is passed to named slots using slot attribute. Example: <div slot='header'>Header content</div>. Must match...
- What are slot props?
  
  Slot props pass data from child to parent through slots. Use let:propertyName directive. Example: <slot name='item'...
- How do you check if slot has content?
  
  Use $$slots object to check slot content. Example: {#if $$slots.header}. Available in component script and template....
- What is slot fallback content?
  
  Fallback content appears when slot is empty. Defined between slot tags. Example: <slot>Fallback</slot>. Provides...
- How do slots work with component composition?
  
  Slots enable flexible component composition. Support content injection at multiple points. Allow component reuse...
- What is the scope of slot content?
  
  Slot content maintains parent component scope. Can access parent variables and functions. Cannot directly access...
- How do you style slot content?
  
  Slot content can be styled in both parent and child. Child styles using :slotted() selector. Parent styles apply...
- What are slots in Svelte?
  
  Slots are placeholders in components that allow parent components to pass content. Defined using <slot> element....
- How do you define a default slot?
  
  Default slots provide fallback content. Example: <slot>Default content</slot>. Content appears when parent doesn't...
- How do you pass content to named slots?
  
  Content is passed to named slots using slot attribute. Example: <div slot='header'>Header content</div>. Must match...
- What are slot props?
  
  Slot props pass data from child to parent through slots. Use let:propertyName directive. Example: <slot name='item'...
- How do you check if slot has content?
  
  Use $$slots object to check slot content. Example: {#if $$slots.header}. Available in component script and template....
- What is slot fallback content?
  
  Fallback content appears when slot is empty. Defined between slot tags. Example: <slot>Fallback</slot>. Provides...
- How do slots work with component composition?
  
  Slots enable flexible component composition. Support content injection at multiple points. Allow component reuse...
- What is the scope of slot content?
  
  Slot content maintains parent component scope. Can access parent variables and functions. Cannot directly access...
- How do you style slot content?
  
  Slot content can be styled in both parent and child. Child styles using :slotted() selector. Parent styles apply...
Routing in Svelte (SvelteKit)
- What is SvelteKit routing?
  
  SvelteKit provides file-based routing where files in the routes directory automatically become pages. URLs...
- How do you create a basic route in SvelteKit?
  
  Create a route by adding a +page.svelte file in the routes directory. Example: src/routes/about/+page.svelte becomes...
- What are dynamic routes in SvelteKit?
  
  Dynamic routes use square brackets in file names. Example: [slug]/+page.svelte creates dynamic segment. Parameters...
- How do you access route parameters?
  
  Route parameters accessed through page.params. Example: export let data; const { slug } = data. Available in...
- What is a layout file in SvelteKit?
  
  Layout files (+layout.svelte) provide shared UI for multiple routes. Define common elements like navigation, footer....
- How do you handle navigation in SvelteKit?
  
  Navigation uses <a> tags or programmatic goto function. SvelteKit handles client-side navigation. Supports...
- What are route groups in SvelteKit?
  
  Route groups organize routes using (group) syntax. Don't affect URL structure. Share layouts within groups. Support...
- How do you implement loading data?
  
  Data loading uses +page.js or +page.server.js files. Export load function for data fetching. Returns props for page...
- What is the error page in SvelteKit?
  
  Error page (+error.svelte) handles route errors. Displays when errors occur. Access error details through error...
- How do you handle redirects?
  
  Redirects use redirect function from @sveltejs/kit. Can redirect in load functions or actions. Support...
Context API & Dependencies
- What is the Context API in Svelte?
  
  The Context API allows passing data through the component tree without prop drilling. Uses setContext and getContext...
- How do you set context in Svelte?
  
  Context is set using setContext function from svelte. Example: setContext('key', value). Must be called during...
- How do you get context in Svelte?
  
  Context is retrieved using getContext function. Example: const value = getContext('key'). Must use same key as...
- What is context key uniqueness?
  
  Context keys must be unique within component tree. Often use symbols for guaranteed uniqueness. Example: const key =...
- How do you handle context lifecycle?
  
  Context exists throughout component lifecycle. Created during initialization. Available until component destruction....
- What are typical context use cases?
  
  Common uses include theme data, localization, authentication state, shared functionality. Useful for cross-cutting...
- How do you share functions via context?
  
  Functions can be shared through context. Example: setContext('api', { method: () => {} }). Allows child components...
- What is the context scope?
  
  Context is scoped to component and descendants. Not available to parent or sibling components. Multiple instances...
- How do you handle missing context?
  
  getContext returns undefined if context not found. Should handle undefined case. Can provide default values....
- What is context vs stores?
  
  Context is static, set during initialization. Stores are reactive, can change over time. Context good for static...
Bindings & Directives
- What are bindings in Svelte?
  
  Bindings create two-way data flow between DOM elements and variables using bind: directive. Example: <input...
- What are the basic form input bindings?
  
  Basic form bindings include value for text inputs, checked for checkboxes, group for radio/checkbox groups. Example:...
- What is the use: directive?
  
  use: directive attaches actions (reusable DOM node functionality) to elements. Example: <div use:action>. Actions...
- What is the class: directive?
  
  class: directive conditionally applies CSS classes. Example: <div class:active={isActive}>. Shorthand available when...
- How do you bind to custom components?
  
  Custom component binding uses bind: on exported props. Component must export the variable. Example: <CustomInput...
- What is the style: directive?
  
  style: directive sets inline styles conditionally. Example: <div style:color={textColor}>. Can use shorthand when...
- How do you bind to select elements?
  
  Select elements bind using value or selectedIndex. Example: <select bind:value={selected}>. Supports multiple...
- What is the this binding?
  
  this binding references DOM element or component instance. Example: <div bind:this={element}>. Useful for direct DOM...
- What is the bind:group directive?
  
  bind:group groups radio/checkbox inputs. Binds multiple inputs to single value/array. Example: <input type='radio'...
- How do you bind to contenteditable elements?
  
  Contenteditable elements bind using textContent or innerHTML. Example: <div contenteditable...
Server Side Rendering (SSR)
- What is Server-Side Rendering in SvelteKit?
  
  Server-Side Rendering (SSR) generates HTML on the server instead of client. Provides better initial page load, SEO...
- What is hydration in SvelteKit?
  
  Hydration is the process where client-side JavaScript takes over server-rendered HTML. Makes static content...
- How do you handle server-side data loading?
  
  Server-side data loading uses load functions in +page.server.js. Returns data for page rendering. Supports async...
- What is the CSR fallback in SvelteKit?
  
  Client-Side Rendering (CSR) fallback handles cases where SSR fails or is disabled. Uses +page.js instead of...
- How do you disable SSR for a route?
  
  Disable SSR using export const ssr = false in +page.js. Page renders only on client. Useful for browser-specific...
- What are server-only modules in SvelteKit?
  
  Server-only modules run exclusively on server. Use .server.js extension. Cannot be imported by client code. Useful...
- How do you handle SSR errors?
  
  SSR errors handled by error.svelte pages. Support error boundaries. Can provide fallback content. Error details...
- What is streaming SSR?
  
  Streaming SSR sends HTML in chunks as it's generated. Improves Time To First Byte (TTFB). Supports progressive...
- How do you handle environment variables in SSR?
  
  Environment variables accessed through $env/static/private or $env/dynamic/private. Only available server-side. Must...
- What is prerendering in SvelteKit?
  
  Prerendering generates static HTML at build time. Uses export const prerender = true. Improves performance. Suitable...
Testing & Debugging
- What is the recommended testing framework for Svelte?
  
  Vitest is the recommended testing framework for Svelte applications. It's fast, provides good integration with...
- How do you write component tests in Svelte?
  
  Component tests use @testing-library/svelte. Mount components using render method. Test component behavior and DOM...
- What is Svelte Inspector?
  
  Svelte Inspector is a development tool enabled with 'ctrl + shift + i'. Shows component hierarchy, props, state....
- How do you handle asynchronous tests?
  
  Async tests use async/await syntax. Test async operations using act function. Handle promises and timeouts. Example:...
- What are testing matchers in Svelte?
  
  Testing matchers verify component state/behavior. Include toBeInTheDocument, toHaveTextContent, toBeVisible....
- How do you debug store state?
  
  Store state can be debugged using $store syntax. Subscribe to store changes. Log state updates. Use Svelte devtools....
- What is component testing hierarchy?
  
  Testing hierarchy includes unit tests, integration tests, end-to-end tests. Focus on component isolation. Test...
- How do you test event handlers?
  
  Event handlers tested using fireEvent. Simulate user interactions. Verify event outcomes. Example:...
- What is snapshot testing?
  
  Snapshot testing captures component output. Compares against stored snapshots. Detects UI changes. Example:...
- How do you test props?
  
  Props testing verifies component behavior with different props. Test prop types and values. Handle prop updates....
Performance & Optimization
- What makes Svelte performant by default?
  
  Svelte achieves performance through compile-time optimization. Generates vanilla JavaScript with minimal runtime...
- What is code splitting in SvelteKit?
  
  Code splitting automatically breaks application into smaller chunks. Routes loaded on demand. Reduces initial bundle...
- How does Svelte handle reactivity?
  
  Svelte compiles reactive statements into efficient JavaScript. Updates only affected DOM elements. No diffing...
- What is asset optimization in SvelteKit?
  
  Asset optimization includes minification, bundling, compression. Uses Vite for build process. Optimizes images and...
- How do you optimize component rendering?
  
  Component optimization includes proper key usage, avoiding unnecessary updates, using tick() function. Implement...
- What is lazy loading in Svelte?
  
  Lazy loading defers loading of components until needed. Uses dynamic imports. Supports route-level code splitting....
- How do you handle memory leaks?
  
  Memory leak prevention includes proper cleanup in onDestroy, unsubscribing from stores, removing event listeners....
- What is prefetching in SvelteKit?
  
  Prefetching loads route data before navigation. Uses sveltekit:prefetch directive. Improves perceived performance....
- How do you optimize store updates?
  
  Store optimization includes batching updates, using derived stores efficiently, implementing selective updates....
- What is tree-shaking in Svelte?
  
  Tree-shaking removes unused code during build. Reduces bundle size. Supported by default. Works with ES modules....

What is Svelte and how is it different from other frameworks?

Svelte is a compiler that creates reactive JavaScript modules. Unlike React or Vue that do most of their work in the browser, Svelte shifts that work into a compile step that happens when you build your app, resulting in highly optimized vanilla JavaScript with minimal runtime overhead.

How do you create a basic Svelte component?

A Svelte component is created in a .svelte file with three main sections: <script> for JavaScript logic, <style> for component-scoped CSS, and the template section for HTML markup. Styles are automatically scoped to the component.

How do you declare reactive variables in Svelte?

Reactive variables in Svelte are declared using the let keyword in the <script> section. Any variables used in the template are automatically reactive. Example: let count = 0. When count changes, the UI automatically updates.

What is the purpose of the $ syntax in Svelte?

The $ prefix in Svelte is a special syntax that marks a statement as reactive. It automatically re-runs the code whenever any referenced values change. It's commonly used with derived values and store subscriptions.

How do you include conditional rendering in Svelte?

Svelte uses #if, :else if, and :else blocks for conditional rendering. Example: {#if condition}...{:else if otherCondition}...{:else}...{/if}. These blocks can contain any valid HTML or component markup.

How do you create loops in Svelte templates?

Svelte uses the #each block for iteration. Syntax: {#each items as item, index}...{/each}. Supports destructuring, key specification, and else blocks for empty arrays. Example: {#each users as {id, name} (id)}.

What is the purpose of export keyword in Svelte?

The export keyword in Svelte is used to declare props that a component can receive. Example: export let name; Makes the variable available as a prop when using the component: <Component name='value' />.

How do you handle basic events in Svelte?

Events in Svelte are handled using the on: directive. Example: <button on:click={handleClick}>. Supports modifiers like preventDefault using |. Example: <form on:submit|preventDefault={handleSubmit}>.

What is component composition in Svelte?

Component composition in Svelte involves building larger components from smaller ones. Components can be imported and used like HTML elements. Example: import Child from './Child.svelte'; then use <Child /> in template.

How do you add styles to Svelte components?

Styles are added in the <style> block and are automatically scoped to the component. Global styles can be added using :global() modifier. Supports regular CSS with automatic vendor prefixing.

What is reactivity in Svelte?

Reactivity in Svelte is the automatic updating of the DOM when data changes. It's handled through assignments to declared variables and the $: syntax. Svelte's compiler creates the necessary code to update the DOM when dependencies change.

How do you create a writable store in Svelte?

Writable stores are created using writable() from svelte/store. Example: const count = writable(0). Provides methods like set() and update() to modify the store value. Subscribe to changes using subscribe() method.

What are reactive declarations in Svelte?

Reactive declarations use the $: syntax to automatically recompute values when dependencies change. Example: $: doubled = count * 2. They run whenever any referenced values change.

How do you update arrays reactively in Svelte?

Arrays must be updated using assignment for reactivity. Use methods like [...array, newItem] for additions, array.filter() for removals, or array.map() for updates. Assignment triggers reactivity.

What is the difference between writable and readable stores?

Writable stores can be modified using set() and update(), while readable stores are read-only. Readable stores are created using readable() and only change through their start function.

How do you subscribe to stores in Svelte?

Stores can be subscribed to using subscribe() method or automatically in templates using $ prefix. Example: $store in template or store.subscribe(value => {}) in script.

What is auto-subscription in Svelte?

Auto-subscription happens when using $ prefix with stores in components. Svelte automatically handles subscribe and unsubscribe. No manual cleanup needed. Works in template and reactive statements.

How do you handle derived values in Svelte?

Derived values can be created using reactive declarations ($:) or derived stores. They automatically update when dependencies change. Used for computed values that depend on other state.

What is the purpose of the set function in stores?

The set function directly sets a new value for a writable store. Example: count.set(10). Triggers store updates and notifies all subscribers. Used for direct value updates.

How do you update objects reactively in Svelte?

Objects must be updated through assignment for reactivity. Use spread operator or Object.assign for updates. Example: obj = {...obj, newProp: value}. Assignment triggers reactivity.

How do you declare props in Svelte components?

Props are declared using the export keyword in the script section. Example: export let propName. Optional props can have default values: export let propName = defaultValue. Props are passed to components as attributes.

How do you pass props to child components?

Props are passed to child components as attributes in the markup. Example: <ChildComponent propName={value} />. Can use shorthand when prop name matches variable: <ChildComponent {value} />.

What are spread props in Svelte?

Spread props allow passing multiple props using spread syntax. Example: <Component {...props} />. Useful when forwarding many props. All properties of the object become individual props.

How do you handle prop validation in Svelte?

Prop validation can be handled through TypeScript types or runtime checks. Use if statements or assertions in component initialization. Can throw errors for invalid props.

How do components communicate events to parents?

Components dispatch custom events using createEventDispatcher. Parent listens using on:eventName. Example: dispatch('message', data). Events bubble by default.

What is event forwarding in Svelte?

Event forwarding passes events up through components using on:eventName. Component can forward DOM events or custom events. Use on:message|stopPropagation to prevent bubbling.

How do you bind to component props?

Two-way binding on props uses bind:propName. Example: <Input bind:value={name} />. Component must export the prop. Updates flow both ways between parent and child.

What are reactive statements in component communication?

Reactive statements ($:) respond to prop changes. Can trigger side effects or derive values. Example: $: console.log(propName). Runs whenever referenced props update.

How do you pass HTML attributes through to elements?

HTML attributes can be forwarded using $$props or $$restProps. Useful for wrapper components. Example: <div {...$$restProps}>. Passes any unhandled props to element.

What is prop destructuring in Svelte?

Props can be destructured in export statement. Example: export let { name, age } = data. Supports default values and renaming. Makes prop handling more concise.

What are lifecycle methods in Svelte?

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.

What is onMount in Svelte?

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.

What is onDestroy in Svelte?

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.

What is beforeUpdate in Svelte?

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.

What is afterUpdate in Svelte?

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.

How do you handle async operations in onMount?

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

What is the execution order of lifecycle methods?

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.

How do you clean up resources in lifecycle methods?

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

What are lifecycle guarantees in Svelte?

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.

How do lifecycle methods work with SSR?

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

How do you handle DOM events in Svelte?

DOM events in Svelte are handled using the on: directive. Example: <button on:click={handleClick}>. Supports all standard DOM events like click, submit, input, etc. Event handler receives the event object as parameter.

What are event modifiers in Svelte?

Event modifiers customize event behavior using | symbol. Common modifiers include preventDefault, stopPropagation, once, capture. Example: <form on:submit|preventDefault={handleSubmit}>.

How do you create custom events in Svelte?

Custom events are created using createEventDispatcher. Import from svelte, initialize in component, then dispatch events. Example: const dispatch = createEventDispatcher(); dispatch('custom', data);

How do you listen to component events?

Component events are listened to using on:eventName. Parent components can listen to dispatched events. Example: <Child on:custom={handleCustom}>. Events bubble by default.

What is event forwarding in Svelte?

Event forwarding passes events up through components using on:eventName. No handler needed for forwarding. Example: <button on:click>. Useful for bubbling events through component hierarchy.

How do you handle inline event handlers?

Inline handlers can be defined directly in the on: directive. Example: <button on:click={() => count += 1}>. Good for simple operations but avoid complex logic.

What is event delegation in Svelte?

Event delegation handles events at a higher level using bubbling. Attach single handler to parent for multiple children. Use event.target to identify source. Improves performance for many elements.

How do you pass data with custom events?

Data is passed as second argument to dispatch. Example: dispatch('custom', { detail: value }). Accessed in handler through event.detail. Supports any serializable data.

What is the event object in Svelte?

Event object contains event details passed to handlers. Includes properties like target, currentTarget, preventDefault(). Available as first parameter in event handlers.

How do you handle multiple events on one element?

Multiple events use multiple on: directives. Example: <div on:click={handleClick} on:mouseover={handleHover}>. Each event can have its own modifiers and handlers.

What are Svelte stores?

Stores in Svelte are reactive data containers that can be shared across components. They provide a way to manage global state and notify subscribers when data changes. Created using writable(), readable(), or derived().

How do you create a writable store?

Writable stores are created using writable() from svelte/store. Example: const count = writable(0). They provide set() and update() methods to modify values, and subscribe() to react to changes.

What is the store contract in Svelte?

The store contract requires an object with subscribe method that takes a callback and returns unsubscribe function. Any object meeting this contract can be used as a store with $ prefix.

How do you subscribe to store changes?

Store changes can be subscribed to using subscribe method or $ prefix in components. Example: count.subscribe(value => console.log(value)) or use $count directly in template.

What are readable stores?

Readable stores are created using readable() and are read-only. They can only be updated through their start function. Useful for external data sources that components shouldn't modify.

How do you use derived stores?

Derived stores are created using derived() and compute values based on other stores. Example: const doubled = derived(count, $count => $count * 2). Update automatically when source stores change.

What is auto-subscription in Svelte?

Auto-subscription happens when using $ prefix with stores in components. Svelte automatically handles subscribe and unsubscribe. No manual cleanup needed. Works in template and script.

How do you update store values?

Store values can be updated using set() or update() methods. Example: count.set(10) or count.update(n => n + 1). Updates notify all subscribers automatically.

What is store initialization?

Stores are initialized with initial value in creation. Example: writable(initialValue). Can be undefined. Support synchronous and asynchronous initialization.

How do stores work with reactivity?

Stores integrate with Svelte's reactivity system. Changes trigger reactive updates. Work with reactive declarations ($:). Support reactive dependencies tracking.

What are transitions in Svelte?

Transitions in Svelte are built-in animations that play when elements are added to or removed from the DOM. They are added using the transition: directive, with built-in functions like fade, fly, slide, etc.

How do you use the fade transition?

Fade transition is used with transition:fade directive. Example: <div transition:fade>. Can accept parameters like duration and delay. Animates opacity from 0 to 1 or vice versa.

What is the difference between in: and out: directives?

in: and out: directives specify different transitions for entering and leaving. Example: <div in:fade out:fly>. Allows different animations for element addition and removal.

What are transition parameters?

Transition parameters customize animation behavior. Example: transition:fade={{duration: 300, delay: 100}}. Include properties like duration, delay, easing function.

How do you use the fly transition?

Fly transition animates position and opacity. Example: <div transition:fly={{y: 200}}>. Parameters include x, y coordinates, duration, opacity settings.

What is the slide transition?

Slide transition animates element height. Example: <div transition:slide>. Useful for collapsible content. Can customize duration and easing.

How do you use easing functions?

Easing functions define animation progression. Import from svelte/easing. Example: transition:fade={{easing: quintOut}}. Affects animation timing and feel.

What is transition:local modifier?

transition:local restricts transitions to when parent component is added/removed. Prevents transitions during internal state changes. Example: <div transition:fade|local>.

How do you use the scale transition?

Scale transition animates size and opacity. Example: <div transition:scale>. Parameters include start, opacity, duration. Useful for pop-in effects.

What is the draw transition?

Draw transition animates SVG paths. Example: <path transition:draw>. Parameters include duration, delay, easing. Useful for path animations.

What are slots in Svelte?

Slots are placeholders in components that allow parent components to pass content. Defined using <slot> element. Enable component composition and content projection. Basic slots receive any content passed between component tags.

How do you define a default slot?

Default slots provide fallback content. Example: <slot>Default content</slot>. Content appears when parent doesn't provide slot content. Useful for optional content.

What are named slots?

Named slots target specific slot locations using name attribute. Example: <slot name='header'>. Content provided using slot='header' attribute. Allows multiple distinct content areas.

How do you pass content to named slots?

Content is passed to named slots using slot attribute. Example: <div slot='header'>Header content</div>. Must match slot name in component. Can pass any valid HTML or components.

What are slot props?

Slot props pass data from child to parent through slots. Use let:propertyName directive. Example: <slot name='item' let:item>. Enables parent to access child component data.

How do you check if slot has content?

Use $$slots object to check slot content. Example: {#if $$slots.header}. Available in component script and template. Useful for conditional rendering.

What is slot fallback content?

Fallback content appears when slot is empty. Defined between slot tags. Example: <slot>Fallback</slot>. Provides default UI when parent doesn't provide content.

How do slots work with component composition?

Slots enable flexible component composition. Support content injection at multiple points. Allow component reuse with different content. Enable layout component patterns.

What is the scope of slot content?

Slot content maintains parent component scope. Can access parent variables and functions. Cannot directly access child component state. Uses parent component context.

How do you style slot content?

Slot content can be styled in both parent and child. Child styles using :slotted() selector. Parent styles apply normally. Support style encapsulation.

What are slots in Svelte?

How do you define a default slot?

Default slots provide fallback content. Example: <slot>Default content</slot>. Content appears when parent doesn't provide slot content. Useful for optional content.

How do you pass content to named slots?

Content is passed to named slots using slot attribute. Example: <div slot='header'>Header content</div>. Must match slot name in component. Can pass any valid HTML or components.

What are slot props?

Slot props pass data from child to parent through slots. Use let:propertyName directive. Example: <slot name='item' let:item>. Enables parent to access child component data.

How do you check if slot has content?

Use $$slots object to check slot content. Example: {#if $$slots.header}. Available in component script and template. Useful for conditional rendering.

What is slot fallback content?

Fallback content appears when slot is empty. Defined between slot tags. Example: <slot>Fallback</slot>. Provides default UI when parent doesn't provide content.

How do slots work with component composition?

Slots enable flexible component composition. Support content injection at multiple points. Allow component reuse with different content. Enable layout component patterns.

What is the scope of slot content?

Slot content maintains parent component scope. Can access parent variables and functions. Cannot directly access child component state. Uses parent component context.

How do you style slot content?

Slot content can be styled in both parent and child. Child styles using :slotted() selector. Parent styles apply normally. Support style encapsulation.

What is SvelteKit routing?

SvelteKit provides file-based routing where files in the routes directory automatically become pages. URLs correspond to file paths. Supports dynamic routes, nested layouts, and route parameters.

How do you create a basic route in SvelteKit?

Create a route by adding a +page.svelte file in the routes directory. Example: src/routes/about/+page.svelte becomes '/about'. File structure mirrors URL structure.

What are dynamic routes in SvelteKit?

Dynamic routes use square brackets in file names. Example: [slug]/+page.svelte creates dynamic segment. Parameters available through page store. Support multiple dynamic segments.

How do you access route parameters?

Route parameters accessed through page.params. Example: export let data; const { slug } = data. Available in +page.svelte and +page.server.js files.

What is a layout file in SvelteKit?

Layout files (+layout.svelte) provide shared UI for multiple routes. Define common elements like navigation, footer. Support nested layouts. Content injected via <slot>.

How do you handle navigation in SvelteKit?

Navigation uses <a> tags or programmatic goto function. SvelteKit handles client-side navigation. Supports prefetching with data attribute. Maintains scroll position.

What are route groups in SvelteKit?

Route groups organize routes using (group) syntax. Don't affect URL structure. Share layouts within groups. Support multiple groups. Help organize large applications.

How do you implement loading data?

Data loading uses +page.js or +page.server.js files. Export load function for data fetching. Returns props for page component. Supports server-side loading.

What is the error page in SvelteKit?

Error page (+error.svelte) handles route errors. Displays when errors occur. Access error details through error prop. Support custom error handling.

How do you handle redirects?

Redirects use redirect function from @sveltejs/kit. Can redirect in load functions or actions. Support temporary/permanent redirects. Handle authentication redirects.

What is the Context API in Svelte?

The Context API allows passing data through the component tree without prop drilling. Uses setContext and getContext functions. Context is available to component and its descendants. Useful for sharing data/functionality.

How do you set context in Svelte?

Context is set using setContext function from svelte. Example: setContext('key', value). Must be called during component initialization. Value can be any type including functions.

How do you get context in Svelte?

Context is retrieved using getContext function. Example: const value = getContext('key'). Must use same key as setContext. Available in component and child components.

What is context key uniqueness?

Context keys must be unique within component tree. Often use symbols for guaranteed uniqueness. Example: const key = Symbol(). Prevents key collisions between different contexts.

How do you handle context lifecycle?

Context exists throughout component lifecycle. Created during initialization. Available until component destruction. Cannot be changed after initialization. New values require component reinitialization.

What are typical context use cases?

Common uses include theme data, localization, authentication state, shared functionality. Useful for cross-cutting concerns. Avoids prop drilling. Supports component composition.

How do you share functions via context?

Functions can be shared through context. Example: setContext('api', { method: () => {} }). Allows child components to access shared methods. Supports dependency injection pattern.

What is the context scope?

Context is scoped to component and descendants. Not available to parent or sibling components. Multiple instances create separate contexts. Follows component hierarchy.

How do you handle missing context?

getContext returns undefined if context not found. Should handle undefined case. Can provide default values. Consider error handling for required context.

What is context vs stores?

Context is static, set during initialization. Stores are reactive, can change over time. Context good for static values/dependencies. Stores better for changing state.

What are bindings in Svelte?

Bindings create two-way data flow between DOM elements and variables using bind: directive. Example: <input bind:value={text}>. Updates flow both ways, DOM to variable and variable to DOM.

What are the basic form input bindings?

Basic form bindings include value for text inputs, checked for checkboxes, group for radio/checkbox groups. Example: <input bind:value>, <input type='checkbox' bind:checked>.

What is the use: directive?

use: directive attaches actions (reusable DOM node functionality) to elements. Example: <div use:action>. Actions can have parameters. Support cleanup through returned function.

What is the class: directive?

class: directive conditionally applies CSS classes. Example: <div class:active={isActive}>. Shorthand available when variable name matches class name.

How do you bind to custom components?

Custom component binding uses bind: on exported props. Component must export the variable. Example: <CustomInput bind:value>. Supports two-way binding.

What is the style: directive?

style: directive sets inline styles conditionally. Example: <div style:color={textColor}>. Can use shorthand when variable name matches style property.

How do you bind to select elements?

Select elements bind using value or selectedIndex. Example: <select bind:value={selected}>. Supports multiple selection with array binding.

What is the this binding?

this binding references DOM element or component instance. Example: <div bind:this={element}>. Useful for direct DOM manipulation or component method access.

What is the bind:group directive?

bind:group groups radio/checkbox inputs. Binds multiple inputs to single value/array. Example: <input type='radio' bind:group={selected} value='option'>.

How do you bind to contenteditable elements?

Contenteditable elements bind using textContent or innerHTML. Example: <div contenteditable bind:textContent={text}>. Supports rich text editing.

What is Server-Side Rendering in SvelteKit?

Server-Side Rendering (SSR) generates HTML on the server instead of client. Provides better initial page load, SEO benefits. SvelteKit handles SSR automatically with hydration on client-side.

What is hydration in SvelteKit?

Hydration is the process where client-side JavaScript takes over server-rendered HTML. Makes static content interactive. Preserves server-rendered state. Happens automatically in SvelteKit.

How do you handle server-side data loading?

Server-side data loading uses load functions in +page.server.js. Returns data for page rendering. Supports async operations. Data available during SSR and hydration.

What is the CSR fallback in SvelteKit?

Client-Side Rendering (CSR) fallback handles cases where SSR fails or is disabled. Uses +page.js instead of +page.server.js. Provides graceful degradation.

How do you disable SSR for a route?

Disable SSR using export const ssr = false in +page.js. Page renders only on client. Useful for browser-specific functionality. Impacts initial page load.

What are server-only modules in SvelteKit?

Server-only modules run exclusively on server. Use .server.js extension. Cannot be imported by client code. Useful for sensitive operations like database access.

How do you handle SSR errors?

SSR errors handled by error.svelte pages. Support error boundaries. Can provide fallback content. Error details available through error prop.

What is streaming SSR?

Streaming SSR sends HTML in chunks as it's generated. Improves Time To First Byte (TTFB). Supports progressive rendering. Available through Response.body.

How do you handle environment variables in SSR?

Environment variables accessed through $env/static/private or $env/dynamic/private. Only available server-side. Must prefix with VITE_ for client access.

What is prerendering in SvelteKit?

Prerendering generates static HTML at build time. Uses export const prerender = true. Improves performance. Suitable for static content.

What is the recommended testing framework for Svelte?

Vitest is the recommended testing framework for Svelte applications. It's fast, provides good integration with SvelteKit, and supports component testing. Jest can also be used but requires additional configuration.

How do you write component tests in Svelte?

Component tests use @testing-library/svelte. Mount components using render method. Test component behavior and DOM updates. Example: import { render } from '@testing-library/svelte'; const { getByText } = render(Component);

What is Svelte Inspector?

Svelte Inspector is a development tool enabled with 'ctrl + shift + i'. Shows component hierarchy, props, state. Helps debug component structure and data flow. Available in development mode.

How do you handle asynchronous tests?

Async tests use async/await syntax. Test async operations using act function. Handle promises and timeouts. Example: await fireEvent.click(button); await findByText('result');

What are testing matchers in Svelte?

Testing matchers verify component state/behavior. Include toBeInTheDocument, toHaveTextContent, toBeVisible. Provided by @testing-library/jest-dom. Support custom matchers.

How do you debug store state?

Store state can be debugged using $store syntax. Subscribe to store changes. Log state updates. Use Svelte devtools. Monitor store mutations.

What is component testing hierarchy?

Testing hierarchy includes unit tests, integration tests, end-to-end tests. Focus on component isolation. Test component interaction. Verify application flow.

How do you test event handlers?

Event handlers tested using fireEvent. Simulate user interactions. Verify event outcomes. Example: fireEvent.click(button); expect(result).toBe(expected);

What is snapshot testing?

Snapshot testing captures component output. Compares against stored snapshots. Detects UI changes. Example: expect(container).toMatchSnapshot();

How do you test props?

Props testing verifies component behavior with different props. Test prop types and values. Handle prop updates. Verify component rendering.

What makes Svelte performant by default?

Svelte achieves performance through compile-time optimization. Generates vanilla JavaScript with minimal runtime overhead. Uses surgical DOM updates. Eliminates virtual DOM overhead.

What is code splitting in SvelteKit?

Code splitting automatically breaks application into smaller chunks. Routes loaded on demand. Reduces initial bundle size. Uses dynamic imports. Improves load time.

How does Svelte handle reactivity?

Svelte compiles reactive statements into efficient JavaScript. Updates only affected DOM elements. No diffing required. Uses fine-grained reactivity system. Minimizes overhead.

What is asset optimization in SvelteKit?

Asset optimization includes minification, bundling, compression. Uses Vite for build process. Optimizes images and styles. Supports cache headers. Improves load performance.

How do you optimize component rendering?

Component optimization includes proper key usage, avoiding unnecessary updates, using tick() function. Implement efficient loops. Minimize state changes. Use proper event handling.

What is lazy loading in Svelte?

Lazy loading defers loading of components until needed. Uses dynamic imports. Supports route-level code splitting. Improves initial load time. Reduces bundle size.

How do you handle memory leaks?

Memory leak prevention includes proper cleanup in onDestroy, unsubscribing from stores, removing event listeners. Clear intervals/timeouts. Handle component disposal.

What is prefetching in SvelteKit?

Prefetching loads route data before navigation. Uses sveltekit:prefetch directive. Improves perceived performance. Supports hover-based prefetching. Reduces loading time.

How do you optimize store updates?

Store optimization includes batching updates, using derived stores efficiently, implementing selective updates. Minimize store subscribers. Handle update frequency.

What is tree-shaking in Svelte?

Tree-shaking removes unused code during build. Reduces bundle size. Supported by default. Works with ES modules. Eliminates dead code.

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Svelte Interview Questions Your Guide to Success

svelte

What is Svelte and how is it different from other frameworks?

How do you create a basic Svelte component?

How do you declare reactive variables in Svelte?

What is the purpose of the $ syntax in Svelte?

How do you include conditional rendering in Svelte?

How do you create loops in Svelte templates?

What is the purpose of export keyword in Svelte?

How do you handle basic events in Svelte?

What is component composition in Svelte?

How do you add styles to Svelte components?

What is reactivity in Svelte?

How do you create a writable store in Svelte?

What are reactive declarations in Svelte?

How do you update arrays reactively in Svelte?

What is the difference between writable and readable stores?

How do you subscribe to stores in Svelte?

What is auto-subscription in Svelte?

How do you handle derived values in Svelte?

What is the purpose of the set function in stores?

How do you update objects reactively in Svelte?

How do you declare props in Svelte components?

How do you pass props to child components?

What are spread props in Svelte?

How do you handle prop validation in Svelte?

How do components communicate events to parents?

What is event forwarding in Svelte?

How do you bind to component props?

What are reactive statements in component communication?

How do you pass HTML attributes through to elements?

What is prop destructuring in Svelte?

What are lifecycle methods in Svelte?

What is onMount in Svelte?

What is onDestroy in Svelte?

What is beforeUpdate in Svelte?

What is afterUpdate in Svelte?

How do you handle async operations in onMount?

What is the execution order of lifecycle methods?

How do you clean up resources in lifecycle methods?

What are lifecycle guarantees in Svelte?

How do lifecycle methods work with SSR?

How do you handle DOM events in Svelte?

What are event modifiers in Svelte?

How do you create custom events in Svelte?

How do you listen to component events?

What is event forwarding in Svelte?

How do you handle inline event handlers?

What is event delegation in Svelte?

How do you pass data with custom events?

What is the event object in Svelte?

How do you handle multiple events on one element?

What are Svelte stores?

How do you create a writable store?

What is the store contract in Svelte?

How do you subscribe to store changes?

What are readable stores?

How do you use derived stores?

What is auto-subscription in Svelte?

How do you update store values?

What is store initialization?

How do stores work with reactivity?

What are transitions in Svelte?

How do you use the fade transition?

What is the difference between in: and out: directives?

What are transition parameters?

How do you use the fly transition?

What is the slide transition?

How do you use easing functions?

What is transition:local modifier?

How do you use the scale transition?

What is the draw transition?

What are slots in Svelte?

How do you define a default slot?