50 Svelte Interview Questions and Answers

Svelte is a modern JavaScript framework that compiles your code at build time, resulting in highly efficient vanilla JavaScript. Unlike traditional frameworks, Svelte shifts most of its work to the compile step instead of client-side runtime.

Unlike React and Vue which use a Virtual DOM, Svelte compiles components into efficient JavaScript code that directly updates the DOM. This results in smaller bundle sizes and better runtime performance as there's no framework overhead.

Reactive declarations in Svelte are variables prefixed with $: that automatically update when their dependencies change. They help maintain reactive state without explicit state management code, making components more maintainable.

Svelte stores are objects that hold values and notify subscribers when those values change. They provide a centralized way to manage application state outside components. Stores are accessed using the $ prefix and automatically update subscribed components.

Events in Svelte are handled using the on: directive followed by the event name (e.g., on:click). Svelte also provides event modifiers like preventDefault and stopPropagation using the | character (e.g., on:click|preventDefault).

Props in Svelte are component inputs declared using the export keyword. They allow data to be passed from parent to child components, enabling component reusability and maintaining the unidirectional data flow.

Svelte transitions are built-in animations that can be applied to elements when they're added or removed from the DOM. They're implemented using the transition:, in:, and out: directives and can be customized with parameters.

Two-way binding in Svelte is achieved using the bind: directive, allowing synchronization between form inputs and component state. It simplifies form handling by automatically updating variables when input values change.

Conditional rendering in Svelte is done using #if, :else if, and :else blocks. These blocks efficiently add or remove elements from the DOM based on conditions, making it easy to show or hide content dynamically.

Svelte components have lifecycle methods like onMount, onDestroy, and beforeUpdate that execute code at specific points in a component's lifecycle. These are useful for setup, cleanup, and managing side effects.

In Svelte, bind:group is used for radio and checkbox inputs to bind multiple inputs to a single value. It's particularly useful for form handling where multiple options need to be managed with a single state variable.

Slot props allow parent components to receive data from child components through slots using the let:propName directive. This creates a two-way communication channel between parent and child components while maintaining component encapsulation.

The tick() function returns a promise that resolves after pending state changes have been applied to the DOM. It's useful for scenarios where you need to wait for DOM updates before executing certain operations.

The @html directive allows rendering raw HTML content in Svelte templates. However, it should be used cautiously as it can expose XSS vulnerabilities if used with untrusted content.

Component composition in Svelte is achieved through slots, props, and events. Components can be nested, share state, and communicate through custom events while maintaining clean separation of concerns.

$$props provides access to all props passed to a component, while $$restProps contains only the props that weren't explicitly declared. They're useful for creating wrapper components and forwarding props.

Keyed each blocks help Svelte track list items during updates using a unique identifier. By specifying (key) after #each, Svelte can efficiently update lists without unnecessary DOM operations.

Reactive statements in Svelte, marked with $:, automatically recompute when their dependencies change. They can contain complex logic and are useful for derived calculations and side effects.

afterUpdate is a lifecycle function that runs after the DOM has been updated. It's useful for operations that need to access the updated DOM, like scrolling or focusing elements.

Class directives allow dynamic class binding using class:name={condition}. They provide a clean way to toggle classes based on component state without string concatenation.

Custom transitions in Svelte are created by functions that return an object with css() or tick() methods. They allow creating reusable animation logic with customizable parameters.

beforeUpdate is a lifecycle function that runs before the DOM is updated. It's useful for capturing the previous state of the DOM or preparing for upcoming changes.

Store contract violations occur when a store doesn't implement the required subscribe method. Proper error handling and type checking help prevent these issues in custom stores.

Contenteditable binding allows two-way binding with editable HTML elements using bind:textContent or bind:innerHTML. It's useful for implementing rich text editors.

Transition events (introstart, introend, outrostart, outroend) allow tracking the status of animations. They're useful for coordinating complex animation sequences.

Custom actions are functions that run when an element is mounted. They return an object with destroy() for cleanup and update() for handling parameter changes.

Reactive declarations ($:) are Svelte's way of computing derived values, while computed properties in other frameworks require explicit getters/setters. Svelte's approach is more concise and maintainable.

The {#key} block forces re-rendering of its contents when the key value changes. It's useful for resetting component state or triggering animations on value changes.

Dynamic components can be loaded using where component is dynamically imported. This enables lazy loading and conditional component rendering.

Store unsubscription is handled automatically in Svelte components when using the $ prefix. For manual subscriptions, the unsubscribe function should be called in onDestroy.

Svelte's compiler performs various optimizations including dead code elimination, component isolation, and minimal runtime overhead. It generates highly efficient JavaScript that only includes necessary code, resulting in smaller bundle sizes and better performance.

Svelte provides built-in SSR capabilities through SvelteKit. It pre-renders components on the server, hydrates them on the client, and supports dynamic imports. The framework handles state management and routing seamlessly between server and client.

Actions are reusable functions that run when an element is mounted and cleaned up when unmounted. They provide a way to add custom DOM behavior, third-party library integration, and handle complex DOM manipulations without cluttering component logic.

Module context allows sharing state between component instances using the setContext and getContext functions. It creates a hierarchical context that's accessible to child components without prop drilling, similar to React's Context API.

Svelte supports TypeScript through preprocessors and provides type definitions for component props, events, and stores. It includes special syntax for typing props (export let foo: string) and generates appropriate type information during compilation.

Special elements like , , , and provide advanced functionality for dynamic components, self-referential components, and handling document-level events.

Svelte's reactivity system uses compile-time analysis to create dependency graphs and generate efficient update code. It tracks assignments to variables and automatically updates dependent values without requiring explicit subscriptions.

Slots in Svelte support advanced composition patterns including named slots, slot props, and fallback content. They enable flexible component composition and can pass data back to parent components through slot props.

Code-splitting in Svelte is achieved through dynamic imports and the import() function. The compiler automatically generates separate chunks for dynamically imported components, optimizing initial bundle size.

Custom stores extend Svelte's basic store contract (subscribe/set/update) to create specialized state management solutions. They can implement complex logic, derived states, and persistent storage while maintaining reactivity.

Svelte provides the spring and tweened stores for smooth animations, along with the flip animation helper for list transitions. These tools enable complex animations with minimal code and optimal performance.

The event dispatcher system allows components to create and dispatch custom events using createEventDispatcher. It supports event forwarding, bubbling, and custom event detail data.

Derived stores compute values based on other stores using the derived function. They automatically update when source stores change and can combine multiple stores into a single reactive value.

Svelte provides await blocks for handling async operations directly in templates. It supports loading states, error handling, and can be combined with stores for more complex data-fetching patterns.

Svelte batches DOM updates using microtasks to optimize performance. The scheduler ensures consistent state updates and minimal DOM operations while maintaining reactive dependencies.

Svelte includes built-in debugging tools like @debug tag, component inspection, and reactive statement tracking. It integrates with browser DevTools and provides detailed compilation warnings.

Svelte automatically handles component cleanup through its compilation process. It generates code to remove event listeners, unsubscribe from stores, and clean up resources when components are destroyed.

Hydration is the process where Svelte attaches event listeners and state management to server-rendered HTML. It's optimized to minimize client-side JavaScript while maintaining interactive features.

Preprocessors extend Svelte's compilation pipeline to support additional syntax, languages, and transformations. They can modify component source code before compilation for features like TypeScript and SCSS.

State persistence in Svelte can be implemented through custom stores that sync with localStorage/sessionStorage, or through integration with external state management solutions while maintaining reactivity.