TypeScript Fundamentals

1. What is TypeScript?

   • TypeScript is a superset of JavaScript that adds static typing to the language.-It helps catch errors at compile time, improving code maintainability and scalability.

2. Why use TypeScript?

   • Improved code quality: Catches type-related errors early on.
   • Better code maintainability: Enforces type safety, making it easier to understand and refactor code.
   • Improved tooling support: Provides better editor support, such as autocompletion, code navigation, and refactoring tools.
   • Larger codebases: Helps manage complexity in large-scale projects.

3. What are the core principles of TypeScript?

   • Static Typing: Defines types for variables, function parameters, and return values.
   • Type Inference: Infers types automatically in many cases, reducing the need for explicit type annotations.
   • Object-Oriented Programming: Supports classes, interfaces, and inheritance.

4. What is the difference between JavaScript and TypeScript?

   • JavaScript: Dynamically typed, interpreted language.
   • TypeScript: Statically typed, compiled to JavaScript.

5. How does TypeScript help with code maintainability?

   • Catches type errors early on, reducing the risk of runtime errors.
   • Improves code readability and understanding.
   • Makes it easier to refactor code without breaking functionality.

Types

6. What are the basic data types in TypeScript?

   • number, string, boolean, null, undefined, void

7. What are array types in TypeScript?

   • number[], string[], any[]
   • Generic array: Array<T>

8. What are tuple types in TypeScript?

   • Represents an array with a fixed number of elements and known types for each element.

9. What is the any type in TypeScript?

   • Represents a value that can be of any type.
   • Should be used sparingly as it disables type checking.

10. What is the unknown type in TypeScript?

    • Safer than any, as it prevents operations on values of unknown type before their type is checked.

11. What are interfaces in TypeScript?

    • Define the contract for an object, specifying the shape and type of its properties.

12. What are classes in TypeScript?

    • Define objects with properties and methods.
    • Support inheritance and polymorphism.

13. What are enums in TypeScript?

    • Define a set of named constants.

14. What are type aliases in TypeScript?

    • Create a new name for an existing type.

15. What is the never type in TypeScript?

    • Represents values that never occur.
    • Used in functions that always throw exceptions or do not have a return value.

Generics

16. What are generics in TypeScript?

    • Allow you to create reusable components that can work with different types of data.
    • Use the <T> syntax to define generic type parameters.

17. What are the benefits of using generics in TypeScript?

    • Increased code reusability.
    • Improved type safety.
    • Better code maintainability.

Type Inference

18. What is type inference in TypeScript?

    • The compiler automatically infers the type of a variable based on its value.

19. Give an example of type inference.

    • const name = "John"; // Type inferred as string

Union and Intersection Types

20. What are union types in TypeScript?

    • Represent a value that can be one of several types.
    • Use the | operator to combine types.

21. What are intersection types in TypeScript?

    • Represent a value that must satisfy multiple type constraints.
    • Use the & operator to combine types.

Modules

22. How do you import modules in TypeScript?

    • import { moduleName } from './path/to/module';

23. How do you export members from a module in TypeScript?

    • export const variableName = value;
    • export function functionName() { ... }
    • export class ClassName { ... }

Decorators

24. What are decorators in TypeScript?
    • A special kind of declaration that can be attached to classes, methods, accessors, properties, and parameters.
    • Used to add metadata to these elements.

Async/Await

25. How can you use async/await in TypeScript?
    • Write asynchronous code in a more synchronous-like manner.
    • Use the async keyword before a function to make it return a Promise.
    • Use the await keyword to pause execution until a Promise resolves.

Error Handling

26. How do you handle errors in TypeScript?
    • Use try...catch blocks to catch exceptions.
    • Throw custom errors using the throw statement.

Testing

27. How can you write unit tests for TypeScript code?
    • Use testing frameworks like Jest or Mocha.
    • Write tests that cover different scenarios and edge cases.

Advanced Topics

28. What is a conditional type in TypeScript?

    • A type that depends on another type.

29. What is a mapped type in TypeScript?

    • A type that transforms properties of an existing type.

30. What is a discriminated union in TypeScript?

    • A union type where one property is used to distinguish between the different types.

Interview Questions

31. Explain the concept of type safety in TypeScript.
32. How would you define an interface for a user object?
33. How would you create a generic function that takes an array of any type and returns a new array with only the unique elements?
34. Explain the difference between any and unknown types.
35. How would you use the useEffect hook in a React component written in TypeScript?
36. How would you create a custom type guard in TypeScript?
37. What are some best practices for writing TypeScript code?
38. How would you use TypeScript with a Node.js backend?
39. Describe your experience with using TypeScript in a real-world project.
40. What are some of the challenges you have faced while working with TypeScript?
41. How do you stay up-to-date with the latest changes in TypeScript?
42. What are some of your favorite TypeScript libraries or tools?
43. Explain how TypeScript helps with code refactoring.
44. How would you use decorators in a TypeScript project?
45. What are some common TypeScript design patterns?
46. How would you use TypeScript to improve the maintainability of a large React application?
47. Explain the concept of type compatibility in TypeScript.
48. How would you handle asynchronous operations using Promises and TypeScript?
49. Describe your experience with using TypeScript with a build system like Webpack or Rollup.
50. How would you approach debugging type errors in TypeScript code?

I hope these questions are helpful for your TypeScript interview preparation!