Typescript Interview Questions and Answers for freshers

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 down to plain JavaScript, making it compatible with all JavaScript environments.

2. What are the benefits of using TypeScript?

   • Answer: TypeScript offers several benefits: improved code maintainability through static typing, early error detection during development, better code organization and readability with interfaces and classes, and enhanced tooling support with features like autocompletion and refactoring.

3. How does TypeScript handle type checking?

   • Answer: TypeScript uses a type system to verify the types of variables, function parameters, and return values. It performs type checking during compilation, reporting errors if type mismatches are found. This helps catch potential bugs before runtime.

4. Explain the concept of interfaces in TypeScript.

   • Answer: Interfaces define the shape of an object, specifying the types of its properties and methods. They are used to enforce a contract on objects, ensuring that they have the expected structure. Interfaces don't provide implementation; they only define the structure.

5. What is 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 they are defined in. `var` is function-scoped, meaning it's accessible within the entire function it's defined in. `const` declares a constant whose value cannot be reassigned after initialization. `let` declares a variable whose value can be reassigned.

6. Explain type aliases in TypeScript.

   • Answer: Type aliases create new names for existing types. They improve code readability and maintainability by providing more descriptive names for complex types. They don't create new types; they just provide alternative names.

7. 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 placeholders for types that are specified later when the component is used.

8. Explain the concept of enums in TypeScript.

   • Answer: Enums define a set of named constants. They improve code readability and maintainability by providing meaningful names for numeric values. They help avoid magic numbers in your code.

9. How do you define a class in TypeScript?

   • Answer: Classes are blueprints for creating objects. They define properties and methods that objects of that class will have. You define a class using the `class` keyword, followed by the class name, and then the class members (properties and methods) within curly braces.

10. What are access modifiers in TypeScript (public, private, protected)?

    • 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 itself. `protected` members are accessible within the class and its subclasses.

11. What are namespaces in TypeScript?

    • Answer: Namespaces are used to organize code into logical units, preventing naming conflicts. They help create a modular structure for large projects.

12. Explain type assertion in TypeScript.

    • Answer: Type assertion tells the TypeScript compiler to treat a value as a specific type, even if the compiler isn't sure. It's used when you know the type better than the compiler, but use with caution as it can bypass type checking.

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

    • Answer: The `any` type disables type checking for a variable. Use it sparingly, ideally only when interacting with legacy JavaScript code or external libraries where types are unknown. Overuse defeats the purpose of using TypeScript.

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

    • Answer: The `unknown` type is safer than `any`. While `any` bypasses type checking completely, `unknown` requires explicit type checks before accessing its properties or methods. This helps prevent runtime errors.

15. Explain optional properties in TypeScript interfaces and types.

    • Answer: Optional properties are marked with a question mark (`?`). They indicate that a property might or might not be present in an object that conforms to the interface or type.

16. What are union types in TypeScript?

    • Answer: Union types allow a variable to hold values of multiple types. They are defined using the pipe symbol (`|`). For example, `let x: string | number;` means `x` can be either a string or a number.

17. What are intersection types in TypeScript?

    • Answer: Intersection types combine multiple types into a single type that includes all members of the combined types. They are defined using the ampersand symbol (`&`). For example, `let x: Person & Student;` means `x` must have all the properties of both `Person` and `Student`.

18. Explain tuple types in TypeScript.

    • Answer: Tuple types represent arrays with a fixed number of elements and known types for each element. For example, `let x: [string, number];` defines a tuple with a string at index 0 and a number at index 1.

19. What are mapped types in TypeScript?

    • Answer: Mapped types allow you to create new types based on existing types by iterating over the properties of the existing type and applying a transformation to each property. They are useful for creating types with modified properties (e.g., making all properties optional).

20. What are conditional types in TypeScript?

    • Answer: Conditional types allow you to define types based on conditions. They are particularly useful for creating types that depend on generic type parameters.

21. Explain the concept of type inference in TypeScript.

    • Answer: Type inference is the ability of the TypeScript compiler to automatically determine the type of a variable based on its value or usage. This reduces the need for explicit type annotations.

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

    • Answer: You can use optional properties, union types (`string | null | undefined`), or type guards to handle null and undefined values. The strictNullChecks compiler option can help catch potential null and undefined errors.

23. What are type guards in TypeScript?

    • Answer: Type guards are functions that refine the type of a variable at runtime. They are used to improve type checking when dealing with union types or values that might be null or undefined.

24. Explain the difference between `==` and `===` in TypeScript (and JavaScript).

    • Answer: `==` performs loose equality comparison, performing type coercion if necessary. `===` performs strict equality comparison, requiring both value and type to be the same. `===` is generally preferred for its predictability.

25. How do you declare a function in TypeScript? Explain the use of function parameters and return types.

    • Answer: Functions are declared using the `function` keyword, specifying parameters with their types and a return type using a colon followed by the return type.

26. How do you use arrow functions in TypeScript?

    • Answer: Arrow functions provide a concise syntax for declaring functions. They have implicit return statements and don't have their own `this` binding.

27. Explain function overloading in TypeScript.

    • Answer: Function overloading allows you to define multiple function signatures with the same name but different parameter types. The compiler selects the appropriate signature based on the arguments provided.

28. What are decorators in TypeScript?

    • Answer: Decorators are a way to add metadata to classes, methods, or properties. They can be used to modify the behavior of the decorated element or add functionality.

29. How do you use modules in TypeScript?

    • Answer: Modules are used to organize code into reusable units. You can use `import` and `export` statements to import and export functionality between modules.

30. Explain the difference between `import` and `require` in TypeScript (and Node.js).

    • Answer: `import` is the standard ES module import syntax used in modern JavaScript and TypeScript. `require` is the CommonJS module syntax used in Node.js. `import` offers better static analysis and tree-shaking capabilities.

31. How do you compile TypeScript code?

    • Answer: You use the TypeScript compiler (tsc) to compile TypeScript code into JavaScript. The compiler checks for type errors and generates JavaScript code that can run in any JavaScript environment.

32. How do you use TypeScript with React?

    • Answer: TypeScript can be used with React to add type safety to React components and props. You can define interfaces or types for components and their props, improving code readability and maintainability.

33. How do you use TypeScript with Angular?

    • Answer: Angular is built with TypeScript, so type safety is built-in. You use TypeScript to define components, services, and other parts of your Angular application.

34. What are some common TypeScript linting tools?

    • Answer: ESLint with TypeScript plugins is a popular choice for linting TypeScript code. It helps enforce coding style and catch potential errors.

35. How do you handle asynchronous operations in TypeScript?

    • Answer: You can use `async` and `await` keywords to write asynchronous code in a more synchronous style. You also need to handle Promises and potentially use `Promise.all` for concurrent operations.

36. What are some common TypeScript design patterns?

    • Answer: Many common design patterns (like Singleton, Factory, Observer, etc.) are applicable and often improved with the type safety that TypeScript provides.

37. How do you test TypeScript code?

    • Answer: You can use various testing frameworks like Jest, Mocha, or Jasmine to test your TypeScript code. These frameworks often integrate well with TypeScript compilers and provide type safety during testing.

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