Deno Interview Questions and Answers for freshers

1. What is Deno?

   • Answer: Deno is a JavaScript and TypeScript runtime built on V8, Chromium's JavaScript engine. It's designed to be a modern alternative to Node.js, emphasizing security, modularity, and ease of use. It uses a built-in TypeScript compiler, supports ES modules natively, and offers built-in tooling like a dependency manager and a testing framework.

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

   • Answer: Key differences include Deno's use of ES modules, its built-in TypeScript support, enhanced security (requiring explicit permissions), a built-in dependency manager (no `package.json` or `npm`), and a focus on a more modern and secure development experience. Node.js relies on CommonJS modules, requires separate TypeScript compilation, and handles security through external tools and practices.

3. How does Deno handle dependencies?

   • Answer: Deno uses URLs to import dependencies directly from the network or local files. This eliminates the need for a `package.json` or `npm` and simplifies dependency management. It downloads and caches dependencies as needed.

4. Explain Deno's security model.

   • Answer: Deno operates in a sandboxed environment by default, restricting access to system resources like the network, file system, and environment variables. Code requires explicit permissions using flags like `--allow-net`, `--allow-read`, and `--allow-write` to access these resources. This enhances security by minimizing the attack surface.

5. How do you import modules in Deno?

   • Answer: Modules are imported using ES module syntax, typically with URLs: `import { someFunction } from "https://example.com/module.ts";`

6. What is `deno run`?

   • Answer: `deno run` is a command-line tool used to execute Deno scripts. It downloads and caches dependencies automatically and runs the specified script.

7. What is `deno fmt`?

   • Answer: `deno fmt` formats your code according to Deno's style guide, ensuring consistency in your codebase.

8. What is `deno lint`?

   • Answer: `deno lint` performs static analysis of your code, identifying potential issues and style violations before runtime.

9. What is `deno test`?

   • Answer: `deno test` executes tests written using Deno's built-in testing framework. It helps ensure code quality and reliability.

10. How do you handle asynchronous operations in Deno?

    • Answer: Deno uses Promises and async/await for handling asynchronous operations, similar to other JavaScript runtimes. This provides a clean and readable way to manage concurrent tasks.

11. Explain the role of the `--allow-*` flags in Deno.

    • Answer: `--allow-*` flags grant Deno specific permissions to access system resources. Examples include `--allow-net` for network access, `--allow-read` for file reading, and `--allow-write` for file writing. These flags are crucial for security as they prevent unauthorized access.

12. How does Deno handle errors?

    • Answer: Deno uses standard JavaScript error handling mechanisms (try...catch blocks). Errors are thrown as exceptions, and they can be caught and handled accordingly.

13. What are the benefits of using TypeScript with Deno?

    • Answer: TypeScript provides static typing, which helps catch errors at compile time, improves code maintainability, and enhances developer productivity. Deno's built-in TypeScript support simplifies the development workflow.

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

    • Answer: Deno provides built-in modules for creating HTTP servers. A simple server can be created using the `serve` module. For example: `import { serve } from "https://deno.land/std@0.192.0/http/server.ts"; const server = serve({ port: 8000 }); console.log("Server started on port 8000"); for await (const req of server) { req.respond({ body: "Hello, World!" }); }`

15. How can you read a file in Deno?

    • Answer: Use the `Deno.readFile` function. Remember to use `--allow-read` flag when running the script. Example: `const data = await Deno.readFile("./myFile.txt"); const decoder = new TextDecoder("utf-8"); const text = decoder.decode(data); console.log(text);`

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

    • Answer: Use the `Deno.writeFile` function. Remember to use the `--allow-write` flag when running the script. Example: `await Deno.writeFile("./myFile.txt", new TextEncoder().encode("Hello, Deno!"));`

17. Explain the concept of top-level await in Deno.

    • Answer: Top-level await allows you to use `await` directly in the top-level scope of a module, making it easier to perform asynchronous operations before the module's execution begins. This is particularly helpful for fetching data or performing other setup tasks before starting the main application logic.

18. What are some of the standard modules available in Deno?

    • Answer: Deno offers a wide range of standard modules, including modules for HTTP, file system operations, encoding, testing, and more. These are usually located at URLs under `https://deno.land/std`. Examples include `http`, `fs`, `path`, and `testing`.

19. How do you handle environment variables in Deno?

    • Answer: Deno provides `Deno.env.get()` to retrieve environment variables. Remember to use the `--allow-env` flag when running the script. Example: `const port = Deno.env.get("PORT") || "8000";`

20. What are some of the advantages of using Deno for web development?

    • Answer: Deno's built-in TypeScript support, strong security model, and efficient module system can contribute to building robust and secure web applications. Its ease of use and modern features make development more streamlined.

21. What are some of the limitations of Deno?

    • Answer: Although Deno is gaining popularity, its ecosystem is still relatively smaller compared to Node.js. Finding third-party libraries might sometimes be more challenging. The reliance on URLs for dependency management might also pose issues in offline environments. Also, some developers may find the security restrictions, although beneficial, to be initially restrictive.

22. What is the difference between `Deno.readTextFile` and `Deno.readFile`?

    • Answer: `Deno.readFile` reads a file as a Uint8Array (raw bytes), while `Deno.readTextFile` reads and decodes the file as a string using UTF-8 encoding, simplifying text processing.

23. How do you create a simple test in Deno?

    • Answer: Use the `Deno.test` function. Example: `Deno.test("My test", () => { assertEquals(1 + 1, 2); });`

24. What is the purpose of the `--unstable` flag in Deno?

    • Answer: The `--unstable` flag enables access to experimental features that are not yet fully stable or supported. Use with caution as these features might change without notice.

25. How can you use a third-party module in Deno from a GitHub repository?

    • Answer: You import it directly using the raw Git URL, specifying the file path. For example, if a module `myModule.ts` is located in a GitHub repository: `import { myFunction } from "https://raw.githubusercontent.com/user/repo/main/myModule.ts";`

26. What are some common use cases for Deno?

    • Answer: Deno is suitable for various applications, including web servers, command-line tools, microservices, and backend APIs, especially where security and a modern development experience are critical.

27. How can you handle exceptions in Deno using try-catch blocks?

    • Answer: Similar to standard JavaScript, use `try...catch` blocks to handle potential errors. Example: `try { // Code that might throw an error } catch (error) { console.error("An error occurred:", error); }`

28. Explain the concept of type guards in TypeScript within a Deno context.

    • Answer: Type guards help refine the type of a variable at runtime. They are useful for narrowing down types based on conditions and improving type safety within TypeScript projects in Deno.

29. How can you use decorators in TypeScript within a Deno project?

    • Answer: Decorators provide a way to modify or enhance classes, methods, or properties using a function. They're supported in Deno via its TypeScript compiler.

30. What are some best practices for writing secure Deno applications?

    • Answer: Always use the principle of least privilege, granting only necessary permissions (`--allow-*` flags). Validate all user inputs, sanitize data, and regularly update Deno and its dependencies. Avoid using outdated or unmaintained modules.

31. How do you debug a Deno application?

    • Answer: You can use browser developer tools (like Chrome DevTools) for debugging Deno applications running in a browser environment. For other applications, `console.log` statements and the use of debuggers in your IDE are common strategies.

32. What is the role of the `Deno.permissions` API?

    • Answer: The `Deno.permissions` API allows you to check the runtime permissions granted to your Deno script. This helps in dynamically adapting behavior based on the available access rights.

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

    • Answer: Within your HTTP request handler, check the `req.method` property to determine the HTTP method used and branch your logic accordingly.

34. Explain how to use the `Deno.args` property.

    • Answer: The `Deno.args` property provides access to command-line arguments passed when running the Deno script. This allows you to make your scripts more versatile and configurable.

35. What is the purpose of the `--reload` flag in Deno?

    • Answer: The `--reload` flag forces Deno to re-download and re-cache remote modules, useful when you have updated dependencies.

36. How do you handle file uploads in a Deno HTTP server?

    • Answer: This requires parsing the request body (usually a `multipart/form-data` encoded body) to extract the file data. Libraries can help simplify this process.

37. How can you create and use custom types in TypeScript within Deno?

    • Answer: Define custom types using `interface`, `type` aliases, or `enum` keywords. These provide better type safety and code clarity.

38. What is the difference between `import` and `export` in Deno's ES module system?

    • Answer: `import` brings external modules into the current module, while `export` makes parts of the current module available for other modules to import.

39. Describe how to use promises and async/await together in Deno.

    • Answer: Promises represent asynchronous operations, and `async/await` provide a more synchronous-looking way to manage them, improving code readability.

40. How can you efficiently manage large files in Deno?

    • Answer: Use streaming techniques to avoid loading the entire file into memory at once. Utilize asynchronous file operations to prevent blocking.

41. Explain how to use the `Deno.connect` API.

    • Answer: `Deno.connect` establishes a TCP connection to a remote host and port. Remember to use the `--allow-net` flag for network access.

42. How can you implement authentication in a Deno web application?

    • Answer: Several methods exist, including basic authentication, token-based authentication (JWTs), or using OAuth 2.0.

43. What are some good resources for learning more about Deno?

    • Answer: The official Deno website, the Deno documentation, and online tutorials and courses are good places to start.

44. How do you handle different HTTP status codes in a Deno HTTP server?

    • Answer: Return appropriate HTTP status codes using the `status` property in the response object (`req.respond({ status: 404, body: "Not Found!" });`).

45. Explain the concept of modules and how they contribute to code organization in Deno.

    • Answer: Modules promote code reusability and organization by breaking down the codebase into smaller, manageable units, making it easier to maintain and scale.

46. What are some tips for improving the performance of Deno applications?

    • Answer: Optimize database queries, minimize unnecessary network requests, use efficient algorithms, and profile your application to pinpoint performance bottlenecks.

47. Describe how to deploy a Deno application to a server.

    • Answer: Deploying typically involves setting up a server environment, copying the application files, and configuring a web server (like Nginx or Apache) to serve your application.

48. How can you integrate Deno with a database (e.g., PostgreSQL, MongoDB)?

    • Answer: Use database drivers (often available as npm packages that can be used with Deno through a cdn) to interact with the database. You'll need to manage connections and execute queries.

49. Explain how to use a type definition file (.d.ts) with a third-party module in Deno.

    • Answer: Provide the type definition file alongside the JavaScript module.

50. How do you handle CORS (Cross-Origin Resource Sharing) issues in a Deno server?

    • Answer: Properly configure CORS headers in the HTTP response to allow requests from specific origins.

51. What are some ways to improve the readability and maintainability of Deno code?

    • Answer: Use consistent naming conventions, add comments to explain complex logic, break down functions into smaller, more focused units, and utilize TypeScript's type system for improved clarity.

52. How can you implement logging in a Deno application?

    • Answer: Use the `console.log` function or a more sophisticated logging library for structured logging and better management of log messages.

53. Describe a scenario where you would choose Deno over Node.js for a project.

    • Answer: Deno's built-in security features and modern TypeScript integration make it a strong choice for projects requiring enhanced security and a cleaner development process, especially when starting from scratch.

54. How can you efficiently handle large amounts of data in a Deno application?

    • Answer: Techniques include data streaming, efficient database interactions, using appropriate data structures, and employing parallel processing when applicable.

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