Typescript Interview Questions and Answers

1. What is TypeScript?

   • Answer: TypeScript is a superset of JavaScript that adds optional static typing. This means you can write JavaScript code with type annotations, allowing the compiler to catch errors before runtime. It compiles to plain JavaScript, making it compatible with any JavaScript environment.

2. What are the benefits of using TypeScript?

   • Answer: Benefits include improved code maintainability (due to static typing), early error detection, better code organization, improved tooling support (like IntelliSense), and enhanced developer productivity.

3. Explain the concept of interfaces in TypeScript.

   • Answer: Interfaces define the shape of an object, specifying the types of its properties and methods. They help enforce consistency and provide a blueprint for objects. They are primarily used for compile-time type checking.

4. What are classes in TypeScript?

   • Answer: Classes are blueprints for creating objects. They encapsulate data (properties) and methods that operate on that data. TypeScript classes support inheritance and other object-oriented programming concepts.

5. How do you define a function in TypeScript? Provide an example with type annotations.

   • Answer: function add(x: number, y: number): number { return x + y; } This defines a function named `add` that takes two numbers as input and returns a number.

6. What are generics in TypeScript?

   • Answer: Generics allow you to write reusable code components that can work with different types without losing type safety. They are defined using angle brackets (e.g., `function identity(arg: T): T { return arg; }`).

7. Explain type aliases in TypeScript.

   • Answer: Type aliases create new names for existing types. They improve code readability and make it easier to manage complex types. For example: `type StringArray = string[];`

8. What is the difference between `let`, `const`, and `var` in TypeScript?

   • Answer: `let` and `const` are block-scoped, while `var` is function-scoped. `const` declares a read-only variable, while `let` allows reassignment. `var` should generally be avoided in modern TypeScript code.

9. Explain the concept of union types in TypeScript.

   • Answer: Union types allow a variable to hold one of several types. For example: `let value: string | number;` means `value` can be either a string or a number.

10. Explain the concept of intersection types in TypeScript.

    • Answer: Intersection types combine multiple types into a single type. An object of an intersection type must have all the properties of all the intersected types. For example: `type Person = { name: string } & { age: number };`

11. What is type assertion in TypeScript?

    • Answer: Type assertion tells the compiler that you know better than it does about the type of a variable. It's done using angle brackets `<>` or `as` keyword. Use with caution!

12. What is a tuple in TypeScript?

    • Answer: A tuple is a fixed-length array where each element has a specified type. For example: `let myTuple: [string, number] = ['hello', 5];`

13. Explain the `any` type in TypeScript. When should you use it (and when shouldn't you)?

    • Answer: `any` disables type checking for a variable. Use it sparingly, typically only when dealing with external libraries or legacy code where type information is unavailable. Overuse defeats the purpose of using TypeScript.

14. What is the `unknown` type in TypeScript? How is it different from `any`?

    • Answer: `unknown` is a safer alternative to `any`. It represents a value of any type, but you can't perform any operations on it until you've narrowed down its type using type guards or type assertions.

15. What are type guards in TypeScript? Give an example.

    • Answer: Type guards are functions that narrow down the type of a variable. They often use `typeof` or `instanceof` checks. Example: `function isString(value: any): value is string { return typeof value === 'string'; }`

16. What are optional parameters in TypeScript functions?

    • Answer: Optional parameters are function parameters that can be omitted when calling the function. They are denoted with a question mark `?` after the parameter name (e.g., `function greet(name?: string) { ... }`)

17. How do you handle default parameters in TypeScript functions?

    • Answer: Default parameters are assigned a default value if the caller does not provide a value for the parameter. Example: `function greet(name: string = 'Guest') { ... }`

18. Explain access modifiers in TypeScript (public, private, protected).

    • Answer: `public` members are accessible from anywhere. `private` members are only accessible within the class. `protected` members are accessible within the class and its subclasses.

19. What is inheritance in TypeScript? Provide an example.

    • Answer: Inheritance allows a class to inherit properties and methods from another class (parent class). Example: `class Animal { ... } class Dog extends Animal { ... }`

20. What is polymorphism in TypeScript?

    • Answer: Polymorphism allows objects of different classes to be treated as objects of a common type. This is often achieved through interfaces and inheritance.

21. What is an abstract class in TypeScript?

    • Answer: An abstract class cannot be instantiated directly; it serves as a base class for other classes. It can contain abstract methods (methods without implementation).

22. Explain the concept of namespaces in TypeScript.

    • Answer: Namespaces help organize code into logical units, preventing naming conflicts. They are similar to modules but are less commonly used in modern TypeScript.

23. What are modules in TypeScript?

    • Answer: Modules are used to encapsulate code into reusable units. They help improve code organization and maintainability. TypeScript supports both CommonJS and ES modules.

24. How do you import and export modules in TypeScript?

    • Answer: Use `import` and `export` statements. Example: `export function myFunction() { ... }` and `import { myFunction } from './myModule';`

25. What is a mapped type in TypeScript?

    • Answer: Mapped types create new types based on existing types. They allow you to transform the properties of an existing type. They are very useful for creating types that mirror existing structures.

26. Explain conditional types in TypeScript.

    • Answer: Conditional types allow you to create types that depend on conditions. This enables creating more dynamic and flexible type systems.

27. What are discriminated unions in TypeScript?

    • Answer: Discriminated unions are union types where one property (a discriminant) is used to distinguish between the different types in the union. This helps type narrowing and improves type safety.

28. How do you use `keyof` in TypeScript?

    • Answer: `keyof` allows you to get the type of the keys of an object or interface. It's often used with mapped types.

29. How do you use `typeof` in TypeScript?

    • Answer: `typeof` is used to get the type of a value at runtime. It can also be used in type guards.

30. Explain the role of the `tsconfig.json` file.

    • Answer: `tsconfig.json` is a configuration file that controls the TypeScript compiler. It specifies compiler options, such as the target JavaScript version, module system, and more.

31. How do you declare and use enums in TypeScript?

    • Answer: Enums define a set of named constants. Example: `enum Color { Red, Green, Blue }`

32. What is the difference between a declaration merging and a namespace?

    • Answer: Declaration merging combines multiple declarations of the same name (e.g., interfaces, classes, namespaces). Namespaces are a way to group related declarations.

33. Explain the concept of nominal typing in TypeScript.

    • Answer: Nominal typing allows you to distinguish between types even if their underlying structure is the same. It's achieved through interfaces or type aliases.

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

    • Answer: You can use union types (e.g., `string | null | undefined`) or the optional chaining operator (`?.`) to safely access properties of potentially null or undefined values.

35. What are decorators in TypeScript?

    • Answer: Decorators are a way to add metadata to classes, methods, and properties. They're typically used for dependency injection, logging, and other metaprogramming tasks.

36. How do you use generics with interfaces?

    • Answer: You use angle brackets `` to define generic type parameters in interfaces, just like you would with functions.

37. What is the `readonly` modifier in TypeScript?

    • Answer: The `readonly` modifier prevents modification of a property after initialization.

38. Explain the use of `infer` keyword in conditional types.

    • Answer: `infer` is used in conditional types to extract a type from a generic type. It's commonly used with `extends` to create more complex type relationships.

39. How do you use index signatures in TypeScript interfaces?

    • Answer: Index signatures allow you to define interfaces with dynamically named properties. Example: `interface Dictionary { [key: string]: any; }`

40. How do you work with promises in TypeScript?

    • Answer: TypeScript supports promises natively. You can use `async/await` for cleaner asynchronous code and type annotations for better type safety.

41. What is a type-safe function composition in TypeScript?

    • Answer: Type-safe function composition involves chaining functions together while maintaining type safety. This often involves generics to preserve type information throughout the composition.

42. Explain how to define and use a custom type guard.

    • Answer: A custom type guard is a function that checks the type of a variable and returns a boolean value indicating whether the variable is of a specific type. It must have a `value is Type` return type predicate.

43. What are mapped type modifiers?

    • Answer: Mapped type modifiers, such as `-` (remove), `+` (add), `?` (optional), and `readonly`, allow fine-grained control over how mapped types are created and modified. They provide powerful tools for manipulating types.

44. How can you improve the performance of your TypeScript code?

    • Answer: Strategies include using efficient algorithms and data structures, minimizing unnecessary type checks, and optimizing compilation settings in `tsconfig.json`.

45. How do you debug TypeScript code?

    • Answer: Use your IDE's debugger (like those in VS Code), set breakpoints, step through code, inspect variables, and leverage the compiler's error messages.

46. Explain the concept of recursive types in TypeScript.

    • Answer: Recursive types refer to types that refer to themselves, often used to represent tree-like or linked list data structures.

47. What are the common challenges faced when working with TypeScript?

    • Answer: Common challenges include managing complex types, dealing with legacy JavaScript code, understanding type inference limitations, and finding effective strategies for refactoring.

48. How do you handle type inference in complex scenarios?

    • Answer: In complex scenarios, you may need to explicitly specify types using type annotations or generics to guide TypeScript's type inference system.

49. How do you test TypeScript code?

    • Answer: Use testing frameworks like Jest, Mocha, or Jasmine. Write unit tests, integration tests, and end-to-end tests. You can use tools like `ts-jest` to integrate TypeScript with your test runner.

50. How can TypeScript improve your team's development workflow?

    • Answer: TypeScript helps catch errors early, improves code readability and maintainability, and leads to better collaboration through improved code understanding.

51. Describe your experience using TypeScript in a real-world project.

    • Answer: [This requires a personalized answer based on your own experience.]

52. What are some best practices for writing maintainable TypeScript code?

    • Answer: Best practices include consistent naming conventions, using interfaces and types effectively, following SOLID principles (if using OOP), and writing clear and concise code with meaningful comments.

53. Explain how to use TypeScript with React.

    • Answer: Use `create-react-app` with TypeScript support or configure a custom setup using TypeScript, webpack, and Babel.

54. Explain how to use TypeScript with Angular.

    • Answer: Angular projects are built with TypeScript. The framework uses TypeScript extensively.

55. How do you handle large-scale TypeScript projects?

    • Answer: Utilize modular design principles, create clear module boundaries, use a consistent coding style, employ advanced TypeScript features like namespaces or modules for organization, and consider using a monorepo approach if necessary.

56. What are some of the newer features in recent TypeScript versions?

    • Answer: [This answer needs to be updated to reflect the latest TypeScript version. Check the official TypeScript release notes for the most current features.]

57. How do you deal with type compatibility issues in TypeScript?

    • Answer: Employ techniques like type guards, type assertions (with caution), type aliases, and interfaces to resolve conflicts and improve type compatibility. Refactoring might be needed in some cases.

58. Explain the difference between `Partial` and `Record` types.

    • Answer: `Partial` makes all properties of type `T` optional, whereas `Record` creates an object type with keys of type `K` and values of type `V`.

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

    • Answer: Common pitfalls include overusing `any`, neglecting type guards, overlooking type inference limitations, and failing to properly configure `tsconfig.json`.

60. How does TypeScript handle optional properties in objects?

    • Answer: Optional properties are denoted with a question mark (`?`) after the property name. They can be omitted when creating objects of that type, and access should be handled carefully to prevent runtime errors.

61. Describe your approach to learning and staying up-to-date with TypeScript.

    • Answer: [This requires a personalized answer based on your own learning methods.]

62. How do you handle TypeScript errors effectively during development?

    • Answer: Carefully read error messages from the compiler, use your IDE's error highlighting, and refactor your code based on the errors to correct type issues.

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