Typescript Interview Questions and Answers for experienced

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 rather than at runtime. This leads to more robust and scalable applications, especially in larger projects with multiple developers.

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

   • Answer: `let` and `const` are block-scoped, meaning they are only accessible within the block of code (e.g., within an `if` statement or a loop) where they are defined. `var` is function-scoped, meaning it's accessible within the entire function it's declared in. `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? Give an example.

   • Answer: Interfaces define the shape of an object, specifying the types of its properties and methods. They don't provide implementation details; they're purely for defining a contract. For example: `interface Person { name: string; age: number; greet(): void; }` Classes and objects can then implement this interface.

4. What are classes in TypeScript? How do they differ from interfaces?

   • Answer: Classes are blueprints for creating objects. They encapsulate data (properties) and methods that operate on that data. Interfaces only define the structure; classes provide implementation. A class can implement multiple interfaces, but it extends only one class (inheritance).

5. Explain the concept of generics in TypeScript. Provide an example.

   • Answer: Generics allow you to write reusable code components that can work with different types without losing type safety. For example, a generic function could be written to sort an array of any type: `function identity<T>(arg: T): T { return arg; }` This function works with numbers, strings, objects, etc., while still maintaining type checking.

6. What are type aliases in TypeScript? When would you use them?

   • Answer: Type aliases create new names for existing types. They improve readability and maintainability, especially when dealing with complex or frequently used types. For example: `type StringArray = string[];` This makes code cleaner than repeatedly writing `string[]`.

7. What is type inference in TypeScript?

   • Answer: Type inference is TypeScript's ability to automatically determine the type of a variable based on its value or context. This reduces the need for explicit type annotations, making the code less verbose while maintaining type safety.

8. Explain the difference between `any` and `unknown` types.

   • Answer: `any` disables type checking for a variable, essentially treating it like a JavaScript variable. `unknown` represents a type whose value is not yet known; you must perform type checking before accessing its properties or methods to prevent runtime errors. `unknown` is safer than `any`.

9. What are optional properties in TypeScript interfaces and classes?

   • Answer: Optional properties are marked with a question mark (`?`) after the property name. They indicate that a property may or may not be present in an object that conforms to the interface or class. For example: `interface Person { name: string; age?: number; }`

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

    • Answer: You can use the `strictNullChecks` compiler option to enforce null and undefined checks. You can also use optional chaining (`?.`) and nullish coalescing (`??`) operators to handle potential null or undefined values safely.

11. What are mapped types in TypeScript? Provide an example.

    • Answer: Mapped types allow you to create new types based on existing types by iterating over the keys of the original type. For example, you can easily create a read-only version of an interface: `type Readonly<T> = { readonly [P in keyof T]: T[P]; };`

12. Explain conditional types in TypeScript.

    • Answer: Conditional types allow you to define types that depend on the type of a variable or expression. They use a ternary-like syntax to select one type based on a condition: `type IsString<T> = T extends string ? true : false;`

13. What are discriminated unions in TypeScript?

    • Answer: Discriminated unions are a way to combine multiple types with a common property that distinguishes between them. This allows TypeScript to narrow down the type based on the value of the discriminant property, improving type safety.

14. How do you use namespaces in TypeScript? When are they useful?

    • Answer: Namespaces help organize code into logical units, preventing naming conflicts. They are declared using the `namespace` keyword. Namespaces are less commonly used now in favor of modules.

15. Explain modules in TypeScript and how they are used for code organization.

    • Answer: Modules are a way to encapsulate related code into separate files, improving code organization and reusability. They enhance maintainability and prevent namespace pollution.

16. How do you use decorators in TypeScript? Give an example.

    • Answer: Decorators are a way to add metadata to classes, methods, or properties. They use the `@decoratorName` syntax. They are commonly used for dependency injection and aspect-oriented programming.

17. What are mixins in TypeScript and how do they work?

    • Answer: Mixins allow you to reuse code across multiple classes without using inheritance. They often involve functions that add methods and properties to a class.

18. Describe the process of compiling TypeScript code.

    • Answer: TypeScript code is compiled into JavaScript using the TypeScript compiler (`tsc`). The compiler checks for type errors and generates JavaScript code that can be run in browsers or Node.js.

19. How do you handle asynchronous operations in TypeScript?

    • Answer: TypeScript uses promises and async/await to handle asynchronous operations. Promises represent the eventual result of an asynchronous operation, and async/await provides a more synchronous-like style of writing asynchronous code.

20. What are some common TypeScript design patterns?

    • Answer: Many common JavaScript design patterns are applicable to TypeScript, including Singleton, Factory, Observer, Decorator, and others. TypeScript's type system can help enforce the structure and correctness of these patterns.

21. How do you work with external libraries in TypeScript?

    • Answer: You typically use type definitions (`.d.ts` files) to provide type information for external libraries. These files are often available through npm packages like `@types/library-name`.

22. Explain the concept of type guards in TypeScript.

    • Answer: Type guards are functions or expressions that narrow down the type of a variable at runtime. They are useful when working with union types or when you need to check the type of a variable before accessing its properties.

23. How do you test TypeScript code?

    • Answer: You can use various testing frameworks like Jest, Mocha, or Jasmine to test TypeScript code. These frameworks can be used in combination with type checking to ensure both the functionality and type safety of your code.

24. What are some best practices for writing TypeScript code?

    • Answer: Use interfaces and types effectively. Keep types consistent and concise. Use type inference judiciously. Employ generics for reusable code. Favor explicit types over `any`. Write unit tests and utilize a linter.

25. How would you handle errors in TypeScript code, especially in asynchronous operations?

    • Answer: Use `try...catch` blocks to handle errors in synchronous and asynchronous code. In async functions, use `try...catch` within the `async` function or propagate errors using promises' `.catch()` method.

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

    • Answer: `tsconfig.json` is the configuration file for the TypeScript compiler. It specifies compiler options such as the target JavaScript version, module system, whether to include `strictNullChecks`, and other settings.

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

    • Answer: Overusing `any`, neglecting to handle null/undefined properly, inconsistent type definitions, ignoring compiler warnings, and not utilizing TypeScript's features like generics.

28. How can you improve the performance of your TypeScript applications?

    • Answer: Efficient algorithms and data structures, code optimization, minimizing unnecessary computations, and using appropriate tooling like bundlers (Webpack, Parcel) for optimized JavaScript output.

29. Explain how TypeScript integrates with popular JavaScript frameworks like React, Angular, or Vue.js.

    • Answer: These frameworks offer official TypeScript support through type definitions and integrations. They allow you to write type-safe code within the framework's structure, benefitting from the framework's features and TypeScript's type system.

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