Typescript Interview Questions and Answers for 2 years experience

1. What is TypeScript and why would you use it?

   • Answer: TypeScript is a superset of JavaScript that adds optional static typing. We use it to improve code maintainability, readability, and to catch errors during development (compile time) rather than runtime. It's particularly beneficial for large projects and teams where type safety is crucial.

2. Explain the difference between `let`, `const`, and `var` in TypeScript.

   • Answer: `let` and `const` are block-scoped, meaning their scope is limited to the nearest enclosing block (e.g., within an `if` statement or a loop). `var` is function-scoped or globally scoped, depending on where it's declared. `const` declares a constant whose value cannot be reassigned after initialization. `let` declares a variable whose value can be reassigned. `var` is generally avoided in modern TypeScript due to its potential for unexpected behavior related to hoisting.

3. What are interfaces in TypeScript and how are they used?

   • Answer: Interfaces define the shape of an object, specifying the properties and their types. They're used to enforce type checking and ensure consistency in data structures. Interfaces don't provide implementation details; they only define the contract.

4. What are type aliases in TypeScript? How are they different from interfaces?

   • Answer: Type aliases create new names for existing types. They can be used to alias primitive types, union types, tuple types, or even other type aliases. Unlike interfaces, they cannot be extended or implemented. They primarily offer a way to improve code readability and reduce redundancy.

5. Explain the concept of generics in TypeScript.

   • Answer: Generics allow you to write reusable code components that can work with different types without losing type safety. They use type parameters (like `T` or `U`) to represent types that will be specified later when the component is used. This avoids the need for type assertions and improves code flexibility.

6. What are union types and intersection types? Provide examples.

   • Answer: Union types (`|`) represent a value that can be one of several types. Example: `let value: string | number;`. Intersection types (`&`) represent a value that must satisfy all specified types. Example: `interface Person { name: string; } interface Worker { job: string; } let personWorker: Person & Worker;`

7. Describe tuple types in TypeScript.

   • Answer: Tuple types are arrays with a fixed number of elements of known types. Example: `let coords: [number, number] = [10, 20];`

8. Explain optional properties in interfaces and types.

   • Answer: Optional properties are marked with a question mark (`?`). They indicate that a property might not be present in an object that conforms to the interface or type. Example: `interface User { id: number; name?: string; }`

9. How do you handle null and undefined values in TypeScript?

   • Answer: Use the `|` operator to include `null` or `undefined` in a union type. Optional chaining (`?.`) and nullish coalescing (`??`) operators help safely access properties of potentially nullish objects. The `strictNullChecks` compiler option enforces stricter null and undefined handling.

10. What are type guards in TypeScript?

    • Answer: Type guards are functions or expressions that refine the type of a variable at compile time. They help the compiler narrow down the type based on runtime checks, enabling more accurate type checking.

11. Explain function overloading in TypeScript.

    • Answer: Function overloading allows you to define multiple function signatures for the same function name. The compiler selects the appropriate signature based on the arguments provided. This improves code clarity and type safety when a function can accept different argument types.

12. How do you use namespaces in TypeScript?

    • Answer: Namespaces help organize code into logical units, preventing naming conflicts. They are defined using the `namespace` keyword. They are less frequently used now with modules, but can still be helpful in certain scenarios.

13. What are modules in TypeScript?

    • Answer: Modules are a way to organize code into reusable units. They improve code organization, maintainability, and prevent namespace collisions. TypeScript uses the ES modules system by default, enabling import and export statements.

14. How do you use `import` and `export` statements in TypeScript?

    • Answer: `export` is used to make declarations (classes, functions, variables, etc.) available to other modules. `import` is used to bring declarations from other modules into the current module. Example: `export class MyClass {}; import {MyClass} from './mymodule';`

15. Explain the concept of decorators in TypeScript.

    • Answer: Decorators are a way to modify classes, methods, or properties using a function. They provide a concise way to add functionality or metadata to existing code without modifying its core logic. They use the `@decoratorName` syntax.

16. How do you use access modifiers (public, private, protected) in TypeScript classes?

    • Answer: Access modifiers control the visibility and accessibility of class members. `public` members are accessible from anywhere. `private` members are only accessible within the class. `protected` members are accessible within the class and its subclasses.

17. What are abstract classes in TypeScript?

    • Answer: Abstract classes cannot be instantiated directly. They serve as blueprints for subclasses, defining common methods and properties that subclasses must implement. Methods declared in abstract classes are implicitly abstract unless a body is provided.

18. Explain the difference between `extends` and `implements` keywords.

    • Answer: `extends` is used for inheritance—creating a subclass that inherits from a superclass. `implements` is used to implement an interface, ensuring that a class satisfies the contract defined by the interface.

19. How do you work with promises in TypeScript?

    • Answer: Promises represent asynchronous operations. You use `.then()` to handle successful results and `.catch()` to handle errors. `async/await` makes working with promises more readable and synchronous-like.

20. What is the purpose of the `async` and `await` keywords?

    • Answer: `async` declares an asynchronous function that returns a promise. `await` pauses execution within an `async` function until a promise resolves, making asynchronous code easier to read and write.

21. How do you handle errors in TypeScript?

    • Answer: Use `try...catch` blocks to handle exceptions. Promises provide `.catch()` for error handling in asynchronous operations. Consider using custom error classes for better error management.

22. What are mapped types in TypeScript?

    • Answer: Mapped types allow you to create new types based on existing types, transforming their properties. They use the syntax `{[key in K]: Type}` where `K` is the key type of the original type.

23. Explain conditional types in TypeScript.

    • Answer: Conditional types allow you to create types that depend on other types. They use the syntax `T extends U ? X : Y`, selecting type `X` if `T` extends `U` and `Y` otherwise.

24. What are type inference and type guards in relation to each other?

    • Answer: Type inference allows TypeScript to automatically determine the types of variables based on their usage. Type guards help the compiler refine type inferences based on runtime checks, enabling more precise typing in conditional logic.

25. Describe your experience with TypeScript's type system.

    • Answer: [Describe your personal experience with the TypeScript type system, highlighting specific types, features, and problem-solving you've undertaken.]

26. How do you handle complex data structures in TypeScript?

    • Answer: [Describe your approach, mentioning interfaces, type aliases, generics, and other relevant techniques.]

27. How do you ensure type safety in asynchronous operations in TypeScript?

    • Answer: [Discuss your approach, including the use of promises, async/await, and proper type annotations for async functions.]

28. How do you debug TypeScript code?

    • Answer: [Describe your debugging workflow, including using the debugger in your IDE, console logging, and using TypeScript's error messages.]

29. Explain your experience with TypeScript in a team environment.

    • Answer: [Describe your collaborative experiences, highlighting aspects like code reviews, adhering to coding styles, and contributing to type definitions.]

30. How have you used TypeScript to improve the maintainability of your projects?

    • Answer: [Explain concrete examples where TypeScript's type system helped in improving maintainability and reducing bugs.]

31. How familiar are you with different TypeScript configuration options (tsconfig.json)?

    • Answer: [Discuss your familiarity with key `tsconfig.json` options, such as `target`, `module`, `strict`, and `esModuleInterop` and how they influence compilation.]

32. What are some common TypeScript compiler errors you've encountered and how did you resolve them?

    • Answer: [Describe several common errors, like type mismatches, unresolved imports, and issues with generics, and how you effectively resolved them.]

33. How do you manage type definitions for external libraries in TypeScript?

    • Answer: [Explain your use of `@types` packages and how you handle situations where type definitions are missing or incomplete.]

34. What are some best practices you follow when writing TypeScript code?

    • Answer: [Outline your best practices, including consistent naming conventions, using interfaces effectively, keeping types concise, and leveraging generics appropriately.]

35. How do you stay up-to-date with the latest developments in TypeScript?

    • Answer: [Describe your methods, including following the official TypeScript blog, participating in online communities, and reading relevant articles and documentation.]

36. Describe a challenging TypeScript problem you encountered and how you solved it.

    • Answer: [Detail a specific challenging situation, explaining your problem-solving approach, and the solution you implemented.]

37. How would you explain the concept of "type safety" to a non-programmer?

    • Answer: [Provide a clear and concise explanation using an analogy to illustrate the benefits of type safety.]

38. What are some common pitfalls to avoid when using TypeScript?

    • Answer: [Discuss potential pitfalls, such as over-engineering types, ignoring compiler warnings, and not effectively utilizing type guards.]

39. How do you balance the benefits of strong typing with the potential for increased development time?

    • Answer: [Discuss your approach to balancing strong typing with development speed, emphasizing a pragmatic approach that prioritizes long-term maintainability.]

40. Are you familiar with any TypeScript testing frameworks?

    • Answer: [Mention relevant testing frameworks like Jest, Mocha, or others, and briefly describe your experience with them.]

41. How do you handle large TypeScript projects with many files and modules?

    • Answer: [Discuss techniques for managing large projects, such as using monorepos, proper module organization, and utilizing build tools effectively.]

42. What is your preferred method for setting up a new TypeScript project?

    • Answer: [Describe your preferred setup process, including using tools like `create-react-app`, `ts-node`, or manual setup and configuration.]

43. How familiar are you with different build systems for TypeScript (e.g., Webpack, Parcel)?

    • Answer: [Describe your experience with any relevant build systems, highlighting your understanding of their role in compiling and bundling TypeScript code.]

44. What are your thoughts on using TypeScript in a JavaScript project incrementally?

    • Answer: [Discuss your approach to gradually integrating TypeScript into existing JavaScript codebases, emphasizing a practical and manageable strategy.]

45. How would you describe your understanding of TypeScript's declaration merging?

    • Answer: [Explain declaration merging and its uses, including potential pitfalls and best practices.]

46. Explain your familiarity with using TypeScript with React, Angular, or Vue.js.

    • Answer: [Describe your experience using TypeScript within a specific framework, highlighting key aspects of type safety within that context.]

47. What are some of the performance considerations when using TypeScript?

    • Answer: [Discuss performance aspects of TypeScript, such as compilation time and runtime overhead, and how to mitigate potential issues.]

48. How do you approach writing well-documented TypeScript code?

    • Answer: [Describe your documentation approach, including JSDoc comments, explaining the purpose and benefits of thorough documentation.]

49. What resources do you find most helpful for learning and staying updated on TypeScript?

    • Answer: [Mention your preferred resources, such as official documentation, blogs, online courses, and community forums.]

50. Describe a situation where you had to refactor existing JavaScript code to use TypeScript.

    • Answer: [Detail a specific instance of refactoring, explaining your approach, the challenges encountered, and the outcome.]

51. How do you determine the appropriate level of type strictness for a given project?

    • Answer: [Discuss your decision-making process regarding type strictness, considering factors like project size, team expertise, and development deadlines.]

52. What are your thoughts on using TypeScript's `any` type?

    • Answer: [Explain your views on the `any` type, highlighting its uses and the importance of using it sparingly to maintain type safety.]

53. How do you handle situations where type inference isn't sufficient and you need to explicitly specify types?

    • Answer: [Describe your approach to explicitly specifying types, discussing when it's necessary and how to do it effectively.]

54. What is your experience with using TypeScript in different project environments (e.g., front-end, back-end, or both)?

    • Answer: [Discuss your experience in various environments, highlighting any differences in approach or challenges.]

55. Are you familiar with TypeScript's support for various JavaScript features (e.g., ES6, ES7, etc.)?

    • Answer: [Describe your understanding of TypeScript's compatibility with different JavaScript versions and the `target` option in `tsconfig.json`.]

56. What's your approach to resolving TypeScript compiler errors that involve complex type relationships?

    • Answer: [Outline your strategies for debugging intricate type errors, including using the compiler's messages, simplifying code, and testing changes incrementally.]

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