Deno Interview Questions and Answers for 2 years experience

1. What is Deno, and how does it differ from Node.js?

   • Answer: Deno is a secure runtime for JavaScript and TypeScript built on V8, Chrome's JavaScript engine. Key differences from Node.js include: built-in TypeScript support, secure by default (requires explicit permissions), uses ES modules, and has a built-in dependency manager (no `npm` or `package.json`). It offers a more modern and secure approach to JavaScript development compared to Node.js's more established ecosystem.

2. Explain Deno's module system. How does it differ from Node.js's CommonJS?

   • Answer: Deno uses ES modules, a standard JavaScript module system. This contrasts with Node.js's traditional CommonJS modules. ES modules offer better tooling support, improved tree-shaking capabilities (removing unused code), and a cleaner module resolution mechanism. They use URLs to import modules, enabling remote imports directly from URLs or local files, making dependency management more straightforward.

3. How does Deno handle dependencies?

   • Answer: Deno doesn't rely on a central package registry like npm. Instead, it directly imports modules via URLs, either from remote repositories (like GitHub or deno.land/x) or local file paths. It caches imported modules for faster subsequent runs. This approach enhances security and simplifies dependency management, avoiding the complexities of package.json and npm.

4. Describe Deno's permission model. Why is it considered more secure than Node.js?

   • Answer: Deno operates with a "secure by default" philosophy. It requires explicit permissions for accessing sensitive resources like the network, file system, environment variables, and more. These permissions are granted using flags when running the script (e.g., `deno run --allow-read --allow-net myScript.ts`). This granular control reduces the risk of malicious code accessing system resources without the user's knowledge, unlike Node.js, which by default has broad access to the system.

5. How do you test a Deno application?

   • Answer: Deno has built-in testing capabilities. You can write tests using the `Deno.test()` function and organize them into test files. The tests are typically located within the same directory as the code being tested. Deno provides utilities to run tests and report results effectively. You can also leverage third-party testing frameworks adapted for Deno.

6. Explain the role of `deno fmt` and `deno lint`.

   • Answer: `deno fmt` automatically formats your Deno code according to a consistent style guide, ensuring code readability and maintainability across a project. `deno lint` analyzes your code for potential style and programming errors, helping you write cleaner and more robust code early in the development process. Both commands contribute to a more professional and standardized coding experience.

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

   • Answer: Deno provides a rich standard library offering modules for various tasks like interacting with the file system (`Deno.readTextFile`, `Deno.writeFile`), networking (`Deno.listen`), HTTP (`Deno.serve`), and more. These modules are readily available and well-documented, reducing the need for external dependencies.

8. How can you handle asynchronous operations in Deno?

   • Answer: Deno uses Promises and async/await for handling asynchronous operations. Promises provide a structured way to manage the execution of asynchronous tasks, and async/await offers a cleaner syntax to write asynchronous code that looks more synchronous. This approach follows modern JavaScript best practices for asynchronous programming.

9. How do you create a simple HTTP server in Deno?

   • Answer: Deno's standard library provides the `Deno.listen` and `Deno.serve` functions for creating HTTP servers. You can handle incoming requests using async functions and respond with appropriate data. This is usually simpler and more direct compared to using third-party frameworks for creating basic servers.

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

    • Answer: Top-level await allows you to use the `await` keyword directly inside a module's top-level scope (outside of any function). This enables performing asynchronous operations before the module's code begins execution, simplifying the handling of initial setup tasks involving network requests or file I/O.

11. How can you use the `--allow-net` flag in Deno?

    • Answer: The `--allow-net` flag grants your Deno script permission to access the network, allowing it to make HTTP requests, connect to other servers, or perform any other network-related operation. Without this flag, network access will be denied for security reasons.

12. What are some of the benefits of using TypeScript in Deno?

    • Answer: Deno's built-in TypeScript support offers several advantages: improved code maintainability through static typing, early detection of errors during development, better code organization, and enhanced collaboration in larger projects. It provides a stronger foundation for writing robust and scalable applications.

13. How do you handle errors in Deno?

    • Answer: Deno uses standard JavaScript error handling mechanisms. You can use `try...catch` blocks to handle exceptions, and the `Error` object provides detailed information about the error that occurred. Deno's standard library also provides specific error types for various situations.

14. Explain the difference between `import` and `export` in Deno's ES modules.

    • Answer: `import` is used to bring in modules from other files or remote URLs, while `export` is used to make parts of your module available for use in other modules. This provides the necessary mechanism for modularizing and reusing code in Deno applications.

15. Describe a scenario where you would prefer Deno over Node.js for a project.

    • Answer: I would prefer Deno over Node.js for a project that requires strong security from the start, utilizes TypeScript extensively, needs a simpler dependency management approach, or benefits from the built-in tooling and standard library that Deno provides. For example, a project requiring secure handling of sensitive user data or one involving many micro-services would benefit from Deno's security model.

16. How do you debug a Deno application?

    • Answer: Deno supports debugging using various tools like the browser's developer tools (for applications that leverage the browser), or using a debugger like VS Code's integrated debugger. You might also use `console.log` for basic debugging.

17. What are some common pitfalls to avoid when working with Deno?

    • Answer: Some common pitfalls include forgetting to specify necessary permissions flags (`--allow-read`, `--allow-net`, etc.), mismanaging asynchronous operations, overlooking the nuances of ES modules, and not utilizing Deno's built-in tooling (like `deno fmt` and `deno lint`).

18. Explain the concept of a task runner in Deno and how you might use one.

    • Answer: While Deno doesn't have a built-in task runner like npm scripts, you can achieve similar functionality using external task runners like `deno run --allow-run ` or by writing your own scripts that automate tasks such as testing, building, or deploying. A task runner streamlines the build process and repetitive tasks.

19. How do you handle environment variables in Deno?

    • Answer: You can access environment variables in Deno using `Deno.env.get("VARIABLE_NAME")`. Remember that you'll need the `--allow-env` flag when running your script to grant permission to access these variables.

20. Discuss your experience working with the Deno standard library's file system APIs.

    • Answer: [Describe your personal experience with file system APIs like `Deno.readTextFile`, `Deno.writeFile`, `Deno.mkdir`, etc. Include specific examples and challenges you faced and how you solved them. This is an opportunity to showcase your practical skills.]

21. How have you utilized TypeScript's type system in your Deno projects?

    • Answer: [Describe how you used types like interfaces, classes, generics, and type aliases to improve code clarity and prevent runtime errors. Give concrete examples from your projects.]

22. How familiar are you with third-party modules within the Deno ecosystem? Give examples.

    • Answer: [Mention any third-party Deno modules you've used, such as Oak (a web framework), and briefly describe their purpose and how they were integrated into your projects.]

23. What are some of the community resources you use to learn about or stay updated with Deno?

    • Answer: [Mention resources such as the official Deno website, Deno's GitHub repository, community forums, or blogs dedicated to Deno development.]

24. Describe a challenging Deno project you worked on and how you overcame the challenges.

    • Answer: [Describe a specific project, highlighting the complexities, problem-solving steps, and the successful outcome. Focus on your problem-solving abilities and technical skills.]

25. How do you ensure code quality in your Deno projects?

    • Answer: [Describe your approach to code quality, mentioning practices like using `deno lint`, writing unit tests, code reviews, following coding style guides, and applying best practices for error handling and asynchronous programming.]

26. What are your preferred methods for version control in Deno projects?

    • Answer: [Describe your experience with Git and how you use branching strategies, commit messages, and pull requests to manage code changes collaboratively.]

27. How familiar are you with different deployment strategies for Deno applications?

    • Answer: [Discuss approaches such as deploying to cloud platforms like AWS, Google Cloud, or deploying to a server using tools like Docker or directly using Deno Deploy.]

28. Explain your understanding of runtime performance considerations in Deno.

    • Answer: [Describe your awareness of factors that impact performance like efficient asynchronous programming, minimizing unnecessary I/O operations, and optimizing code for the V8 engine.]

29. What are some areas in Deno that you are keen to learn more about?

    • Answer: [Mention specific areas within the Deno ecosystem such as advanced features of the standard library, specific third-party modules, or new features released in recent versions.]

30. How do you handle dependencies across different versions of Deno?

    • Answer: [Discuss strategies like using import maps, carefully managing dependencies in your `deno.json` configuration, and testing compatibility across different Deno versions.]

31. How would you approach building a RESTful API using Deno?

    • Answer: [Outline the steps involved, mentioning the use of frameworks like Oak, defining routes, handling requests and responses, and implementing appropriate error handling and data validation.]

32. Describe your approach to writing maintainable and scalable Deno code.

    • Answer: [Discuss strategies like modularity, well-structured code, consistent naming conventions, and the use of TypeScript's type system to enhance maintainability and scalability.]

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