Deno Interview Questions and Answers

1. What is Deno?

   • Answer: Deno is a modern runtime for JavaScript and TypeScript built on V8, the same JavaScript engine as Chrome. It's designed to be a secure, modern, and efficient alternative to Node.js. Key features include built-in TypeScript support, secure by default (no file system access unless explicitly granted), and a modern module system using ES modules.

2. What are the key differences between Deno and Node.js?

   • Answer: Deno uses ES modules, while Node.js traditionally uses CommonJS modules (though ES modules are increasingly supported). Deno is secure by default, requiring explicit permissions for file system access, network access, etc., unlike Node.js. Deno has built-in TypeScript support, while Node.js requires external tools. Deno uses a single executable, while Node.js often requires npm and other dependencies.

3. How do you run a Deno script?

   • Answer: You run a Deno script using the `deno run` command followed by the script's path. For example: `deno run myScript.ts`.

4. Explain Deno's module system.

   • Answer: Deno uses standard ES modules. Modules are imported using URLs, allowing for easy access to remote modules from CDNs or Git repositories. This differs from Node.js's traditional CommonJS system which relies on file paths.

5. How does Deno handle dependencies?

   • Answer: Deno doesn't have a package manager like npm. Dependencies are imported directly via URLs in your code. Deno caches dependencies, so subsequent runs are faster.

6. What are Deno's built-in modules? Give examples.

   • Answer: Deno provides several built-in modules for common tasks. Examples include `std/http` for HTTP servers, `std/fs` for file system operations, `std/log` for logging, and `std/path` for path manipulation. These modules are part of the standard library.

7. How do you handle permissions in Deno?

   • Answer: Deno's security model is based on permissions. By default, Deno denies access to the file system, network, and environment variables. You must explicitly grant these permissions using flags like `--allow-read`, `--allow-net`, `--allow-env` when running the script.

8. What is the purpose of the `deno lint` command?

   • Answer: `deno lint` analyzes your code for style and potential errors, helping maintain code quality and consistency. It helps enforce best practices.

9. Explain Deno's testing capabilities.

   • Answer: Deno has built-in support for testing using the `Deno.test` function. You can write tests directly within your modules or in separate test files.

10. How do you use TypeScript in Deno?

    • Answer: Deno has native TypeScript support. You can write your scripts in `.ts` files, and Deno will compile them to JavaScript automatically before execution. No additional configuration is needed.

11. What is the role of the `deno fmt` command?

    • Answer: `deno fmt` automatically formats your code according to a consistent style guide, improving readability and maintainability.

12. How can you debug a Deno application?

    • Answer: You can use the `--inspect` or `--inspect-brk` flags with `deno run` to enable debugging with Chrome DevTools or other debugging tools that support the Chrome DevTools protocol.

13. Explain the concept of a Deno module in detail.

    • Answer: A Deno module is a JavaScript or TypeScript file that exports functions, classes, or variables that can be imported and used in other modules. Import paths are typically URLs, allowing for remote modules.

14. How are remote dependencies handled in Deno?

    • Answer: Remote dependencies are imported using URLs. Deno fetches the module from the given URL, caches it locally, and then uses the cached version on subsequent runs unless the module has been updated.

15. What is the difference between `import` and `export` in Deno?

    • Answer: `import` is used to bring in modules and their exports into a current module. `export` makes functions, classes, or variables available to other modules that import this one.

16. How can you create a simple HTTP server in Deno?

    • Answer: Use the built-in `std/http` module. A basic server could look like this: `import { serve } from "https://deno.land/std@0.192.0/http/server.ts"; for await (const req of serve({})) { req.respond({ body: new TextEncoder().encode("Hello World") }); }`

17. How do you handle errors in Deno?

    • Answer: Use standard JavaScript `try...catch` blocks to handle exceptions. Deno's errors are generally JavaScript `Error` objects, or instances of more specific error types.

18. How can you read a file in Deno?

    • Answer: Use the `Deno.readFile` function or the `Deno.readTextFile` function from the `std/fs` module, remembering to grant the `--allow-read` permission.

19. How can you write to a file in Deno?

    • Answer: Use the `Deno.writeFile` function or `Deno.writeTextFile` function from the `std/fs` module, ensuring you have the `--allow-write` permission.

20. Explain the concept of asynchronous operations in Deno.

    • Answer: Deno is built on top of an asynchronous event loop. Many operations, like file I/O and network requests, are handled asynchronously using Promises or async/await, preventing blocking.

21. How do you make a network request in Deno?

    • Answer: Use the `fetch` API, similar to how it's used in the browser. Remember to grant the `--allow-net` permission.

22. What are the benefits of using Deno's built-in modules?

    • Answer: They're standardized, well-tested, and part of the core language, reducing external dependencies and improving security. They often provide more efficient implementations than third-party packages.

23. How does Deno handle type checking?

    • Answer: Deno's TypeScript integration handles type checking during compilation. Type errors will be reported during the compilation process, before runtime.

24. What are some common use cases for Deno?

    • Answer: Web servers, command-line tools, microservices, and backend APIs are common applications for Deno. Its simplicity and security make it suitable for various projects.

25. How can you improve performance in a Deno application?

    • Answer: Employ asynchronous operations, use efficient data structures and algorithms, properly cache data, and profile your application to identify performance bottlenecks.

26. What are some potential drawbacks of using Deno?

    • Answer: A relatively smaller community compared to Node.js, fewer readily available third-party libraries, and a slightly steeper learning curve for those unfamiliar with ES modules could be considered drawbacks.

27. How does Deno's security model compare to Node.js?

    • Answer: Deno's security model is significantly more restrictive by default. It requires explicit permissions for potentially dangerous operations, unlike Node.js which has broader default access.

28. What is the future of Deno?

    • Answer: Deno's future looks promising. Continued development, improved tooling, and community growth suggest it will become increasingly popular for modern JavaScript and TypeScript projects.

29. How do you handle environment variables in Deno?

    • Answer: Use `Deno.env.get("VARIABLE_NAME")` to access environment variables. Remember to grant `--allow-env` permission when running.

30. Explain the concept of a "top-level await" in Deno.

    • Answer: Top-level await allows you to use `await` outside of an async function in a Deno script's main execution flow. This simplifies code that needs to perform asynchronous operations before the rest of the program runs.

31. How can you use third-party libraries in Deno?

    • Answer: You import them using their URLs. For instance, `import { someFunction } from "https://example.com/my-library.ts";`

32. How do you create a background task in Deno?

    • Answer: Use `Deno.run` to execute a command in the background, or employ worker threads for more complex scenarios.

33. How can you measure the performance of your Deno code?

    • Answer: Use benchmarking tools, profile your application, or manually measure execution times to determine performance bottlenecks.

34. What are some best practices for writing secure Deno code?

    • Answer: Always use explicit permissions, sanitize user inputs, validate data thoroughly, and keep your dependencies updated to address security vulnerabilities.

35. How do you handle large files efficiently in Deno?

    • Answer: Use stream processing to avoid loading the entire file into memory at once. Process the file chunk by chunk.

36. How does Deno compare to other runtime environments like Bun?

    • Answer: Both Bun and Deno are modern JavaScript/TypeScript runtimes aiming to improve upon Node.js, but they have different approaches to security, module handling, and package management. Bun focuses on speed and simplicity, while Deno prioritizes security and a secure-by-default philosophy.

37. What is the role of the Deno standard library?

    • Answer: The Deno standard library provides a set of built-in modules that offer functionality for common tasks like HTTP, file system access, and networking. This reduces reliance on external dependencies.

38. How do you handle asynchronous errors in Deno?

    • Answer: Asynchronous operations return Promises, which can be handled using `.catch()` to handle rejection. Async/await also provides a cleaner way to handle errors within async functions.

39. Describe how Deno's caching mechanism works.

    • Answer: Deno caches downloaded modules locally to speed up subsequent runs. It checks for updates based on the module's URL and last modified time.

40. How can you deploy a Deno application?

    • Answer: Deployment methods depend on the application's purpose. For servers, options include deploying directly to a server, using containerization (Docker), or using cloud platforms like AWS, Google Cloud, or Azure.

41. Explain the use of the `--unstable` flag in Deno.

    • Answer: The `--unstable` flag enables access to experimental features in Deno that are not yet considered stable and may change in future versions. Use with caution.

42. How do you work with JSON data in Deno?

    • Answer: Use `JSON.parse()` to convert JSON strings into JavaScript objects and `JSON.stringify()` to convert JavaScript objects into JSON strings.

43. How do you handle different HTTP methods (GET, POST, etc.) in a Deno server?

    • Answer: In the HTTP request handler, check the `req.method` property to determine the HTTP method used in the request and handle it accordingly.

44. How can you create a WebSocket server in Deno?

    • Answer: Use a WebSocket library available in Deno's ecosystem. A good starting point is to search for WebSocket modules on sites like deno.land/x.

45. What are some alternatives to Deno for building backend applications?

    • Answer: Node.js, Go, Rust, Python (with frameworks like Flask or Django), and others are popular alternatives for backend development.

46. How do you handle database interactions in Deno?

    • Answer: Use database drivers available through the Deno ecosystem. You'll need to find a suitable driver for your chosen database system and install it using the URL import system.

47. What are some strategies for managing dependencies in large Deno projects?

    • Answer: Organize modules into well-defined packages, use a versioning system for your code and dependencies, and consider using a dependency management tool (though not built into Deno itself).

48. How does Deno's built-in type system impact code maintainability?

    • Answer: The built-in TypeScript support enhances code maintainability by catching type errors during development, improving code readability and reducing runtime errors.

49. How do you integrate Deno applications with other systems or services?

    • Answer: Use standard network protocols (HTTP, WebSockets, etc.), message queues, or APIs depending on the requirements of the integration.

50. Explain the benefits of using a consistent coding style in Deno projects.

    • Answer: Consistent code style increases readability, reduces cognitive load for developers working on the project, and improves maintainability.

51. How can you contribute to the Deno open-source project?

    • Answer: Contribute by reporting bugs, suggesting improvements, writing documentation, or contributing code to the core project or related modules.

Thank you for reading our blog post on 'Deno Interview Questions and Answers'.We hope you found it informative and useful.Stay tuned for more insightful content!