Svelte.js Interview Questions and Answers for experienced

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100 Svelte.js Interview Questions and Answers

  1. What is Svelte.js and how does it differ from other JavaScript frameworks like React or Vue?

    • Answer: Svelte is a radical new approach to building user interfaces. Unlike React or Vue, which do the bulk of their work in the browser, Svelte compiles your code into small, highly optimized vanilla JavaScript modules at build time. This results in smaller bundle sizes, improved performance, and less runtime overhead. React and Vue manipulate the DOM (Document Object Model) directly, leading to more JavaScript execution in the browser. Svelte compiles away this overhead. The key differences lie in the *compilation* process and the resulting application performance and size.

  2. Explain the concept of reactivity in Svelte.

    • Answer: Svelte's reactivity is built into the compiler. When you declare a variable in a Svelte component, Svelte automatically tracks its dependencies. When a dependency changes, Svelte efficiently updates only the parts of the DOM affected by that change. This is done through a sophisticated system of tracking and updating, removing the need for a virtual DOM as seen in React and the similar mechanisms in Vue. This makes Svelte highly efficient.

  3. Describe the role of `$:` in Svelte.

    • Answer: The `$:` symbol is used to create reactive declarations. Anything following `$:` is evaluated whenever a dependency changes. This allows for creating reactive statements without explicitly defining watchers or listeners. The expression after `$:` will run whenever any of its dependencies change, enabling efficient updates to the component.

  4. What are stores in Svelte and how are they used? Give examples of different store types.

    • Answer: Stores are a mechanism for managing and sharing state across multiple components. Svelte offers several built-in store types, including writable, readable, and derived stores. Writable stores allow for direct mutation of data. Readable stores provide a way to access data without modifying it. Derived stores compute values based on other stores. They facilitate centralized state management, making it easier to maintain and update data in larger applications.

  5. Explain the difference between `{#each}` and `{#await}` blocks.

    • Answer: `{#each}` iterates over an array or iterable object, dynamically rendering its contents. `{#await}` handles asynchronous operations, rendering different content based on the promise's state (pending, fulfilled, rejected). `{#each}` is for looping; `{#await}` is for managing asynchronous operations and displaying loading/error states.

  6. How do you handle events in Svelte components? Provide examples.

    • Answer: Events are handled using standard HTML event attributes directly on elements within Svelte components. For example, ``. The `handleClick` function would be defined within the component's script section. Svelte simplifies event handling with its concise syntax.

  7. What are slots in Svelte and how are they used for component composition?

    • Answer: Slots allow you to inject content from a parent component into a child component. This enables creating reusable components with customizable content areas. The `` tag within a child component defines a slot; the parent component places content within `` or `` to populate the slot. Named slots provide more control over content placement within the child component.

  8. Explain Svelte's action system. Give an example of creating and using a custom action.

    • Answer: Svelte actions allow you to add behavior to DOM elements. An action is a function that receives the element as an argument and can manipulate it (e.g., adding event listeners, modifying styles). Custom actions are created by defining a function that takes the node and optionally an unmount function as arguments and returns an object with `update` and `destroy` methods. This provides a clean way to manage DOM manipulation and cleanup.

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