TypeScript interview questions with answers

1. What is TypeScript? Why use TypeScript over JavaScript?

TypeScript is a superset of JavaScript that adds static typing and compiles down to plain JavaScript. It helps catch errors at compile-time rather than runtime. Typescript can be preferred over javascript due to:

Type safety,
Better tooling with IDEs,
Early error detection,
Support for modern JavaScript features.

2. What is the difference between “any”, “unknown”, and “never”?

any disables type checking where as unknown is a type-safe version of any which needs type checking before use. never is for functions that never return (e.g., throw or infinite loop)

3. What are interfaces in TypeScript? What is type inference?

Interfaces define the structure of an object and can be used to define types for objects, classes, and function signatures. TypeScript automatically determines the type of a variable if not explicitly defined.

4. How is TypeScript different from JavaScript?

When compared with Javascript, TypeScript adds optional static typing, interfaces, enums, generics, and compile-time error checking.

5. How do you compile a TypeScript file? What is the tsconfig.json file?

To compile a Typescript file, Use the following command:

tsc filename.ts

tsconfig.json file defines TypeScript compiler options and the root files for the project.

6. What are enums and genetics in TypeScript?

Enums allow defining a set of named constants:

enum Color { Red, Green, Blue }

Generics enable writing reusable and flexible code:

function identity<T>(arg: T): T { return arg; }

7. What is the difference between interface and type alias?

Interface and type alias are used to define types in which Interfaces can be extended, and are more suited for OOP. type can define union/intersection types.

8. What are utility types in TypeScript?

Utility types are Built-in helpers like:

Partial<T>,
Pick<T, K>,
Omit<T, K>,
Readonly<T>,
Record<K, T>.

9. Explain about readonly and tuple in TypeScript.

readonly makes properties immutable after initialization.

Tuple is array with fixed number and types of elements:

let x: [string, number] = ['hello', 10];

10. How do you create a union type and a type-safe API response model?

To create a union type:

type ID = string | number;

Creating a type-safe API response model:

interface ApiResponse<T> { status: number; data: T; }

11. What is type assertion and keyof?

Type assertion is used to tell the compiler the type of a variable:

let x = (y as string);

Keyof gets the keys of a type as a union:

type Person = { name: string, age: number };
type Keys = keyof Person; // 'name' | 'age'

12. How do you extend multiple interfaces and handle dynamic object keys in TypeScript?

To extend multiple interfaces:


interface A { a: string; } 
interface B { b: number; } 
interface C extends A, B { c: boolean; }

Handling dynamic object keys in TypeScript:

const obj: { [key: string]: number } = { x: 1, y: 2 };

13. How do you use Record utility type in TypeScript?

Record utility type: Creates an object type with specific keys and values:


type Roles = 'admin' | 'user'; 
const permissions: Record<Roles, boolean> = { admin: true, user: false };

14. What is declaration merging in TypeScript?

Declaration merging refers to the merging of two declarations with the same name by the compiler. It is useful in extending interfaces or modules:

interface Box { height: number; } interface Box { width: number; } // Box now has both height and width

15. How can you restrict function arguments to only certain object shapes?

Restricting function argument to only certain object shapes are:


interface User { name: string; age: number; } 
function createUser(user: User) { ... }

16. How would you use a generic in a class?

Usage of generic in a class:


class Box<T> { 
  contents: T; 
  constructor(value: T) { this.contents = value; } 
}

17. How to make all properties of a type optional?

To make all properties of a type as optional:

type PartialUser = Partial<User>;

18. How can you enforce immutability in TypeScript objects?

Use readonly keyword:

interface User { readonly id: number; name: string; }

19. How to ensure a class method is implementing an interface method?


interface Animal { speak(): void; } 
class Dog implements Animal { speak() { console.log('Bark'); } }

20. What would you use to exclude certain keys from a type?


type User = { id: number; name: string; password: string; }; 
type PublicUser = Omit<User, 'password'>;

21. What is the difference between == and === in TypeScript?

==: compares values with type coercion.
===: strict comparison without coercion.

Always prefer === in TypeScript for type safety.

22. How do you define optional parameters in TypeScript functions?


function greet(name?: string) 
{ console.log('Hello', name || 'Guest'); }

23. What is a discriminated union in TypeScript?

A union of types that share a common literal property to differentiate between them:


type Shape = 
  | { kind: 'circle'; radius: number } 
  | { kind: 'square'; side: number }; 
function area(shape: Shape) 
{ if (shape.kind === 'circle') 
  return Math.PI * shape.radius ** 2; }

24. How do you handle null or undefined in TypeScript?

Use strictNullChecks compiler option.
Use optional chaining ?. and nullish coalescing ??:

let name = user?.profile?.name ?? ‘Anonymous’;

25. Can you explain typeof and instanceof in TypeScript?

typeof: checks primitive types (string, number, etc.)
instanceof: checks object instances



if (typeof value === 'string') { ... } 
if (obj instanceof MyClass) { ... }
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TypeScript interview questions with answers