TypeScript Interview Questions

TypeScript is a powerful programming language that maintains JavaScript’s flexibility while adding static type checking, advanced IDE support, and a more sustainable software development experience for large projects. It is especially preferred in frontend development, working with modern frameworks like React and Angular.

In this article, we have compiled frequently asked questions in real interviews for candidates preparing for TypeScript interviews. Throughout the content, the following SEO keywords are naturally integrated:
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1. What is TypeScript? How Does It Differ from JavaScript?

TypeScript is a superset of JavaScript that includes a static type system. It is written primarily in .ts files and compiled into JavaScript for execution in browsers. JavaScript is dynamic, while TypeScript is statically typed. This difference provides significant advantages in large team projects.

Why is it important?

• Prevents compile-time errors.
• Improves code readability and maintainability.
• Provides high efficiency in auto-completion and refactoring.

TypeScript example:

function add(a: number, b: number): number {
 return a + b;
}

JavaScript example:

function add(a, b) {
 return a + b;
}

Tip:
When writing TypeScript, try to specify types for every function and variable to improve readability and ease debugging.

2. What is the Type System in TypeScript?

The power of TypeScript comes from its type system. Developers can define types for variables, functions, objects, and classes to build safer software.

type User = {
 name: string;
 age: number;
 email?: string;
};

const person: User = {
 name: "Ali",
 age: 28
};

Key TypeScript Types:

• string, number, boolean
• any, unknown, void, never
• array, tuple
• union, intersection, literal
• enum, type, interface

Quick note:
Avoid using any as much as possible. unknown is a safer alternative because it forces type checking before usage.

3. What Are the Differences Between Interface and Type?

Both interface and type are used to define object structures but differ in usage and capabilities.

interface Person {
 name: string;
 surname: string;
}

type PersonType = {
 name: string;
 surname: string;
};

Differences:

• interface can be extended with extends, while type can be combined using &.
• type can be used with unions and tuples.
• Interfaces are more suitable for defining object shapes; types work better for complex type combinations.

Tip:
In React projects, type is more common, whereas Angular favors interface.

4. How to Use TypeScript with React?

Using TypeScript in React secures props, state, and context structures, making data flow between complex components safer.

type ButtonProps = {
 text: string;
 onClick: () => void;
};

const Button: React.FC<ButtonProps> = ({ text, onClick }) => (
 <button onClick={onClick}>{text}</button>
);

Advantages:

• Provides type safety for component interfaces.
• Auto-completion speeds up coding.
• Reduces runtime errors.

Tip:
Clearly mark required (text) and optional (icon?) props.

5. Using TypeScript in Angular Projects

Angular is built directly on TypeScript. Its components, services, modules, and directives are created with TypeScript.

@Component({
 selector: 'app-example',
 templateUrl: './example.component.html'
})
export class ExampleComponent {
 message: string = "Hello TypeScript!";
}

Benefits of TypeScript with Angular:

• Dependency Injection is more robust.
• Decorators like @Input and @Output gain strong typing.
• Complex apps become more readable.

6. What Are Type Assertion and Type Guard?

Type Assertion: Tells TypeScript you know a variable’s type better than it does.

Type Guard: Runtime type checking techniques.

let data: any = "Hello";
let length: number = (data as string).length;

function check(value: string | number) {
 if (typeof value === "string") {
   console.log(value.toUpperCase());
 }
}

Used Techniques:

• typeof
• instanceof
• in

Tip:
Use type guards to perform type-specific operations in if-blocks—very useful with generics.

7. How to Use Generics?

Generics enable writing reusable, type-safe functions and classes, especially useful for functions like filtering lists.

function firstElement<T>(array: T[]): T {
 return array[0];
}

const numberItem = firstElement<number>([1, 2, 3]);
const stringItem = firstElement<string>(["a", "b"]);

Why is it important?

• Works independently of type.
• Reduces code duplication.
• Essential for component library development.

8. What Are Union and Intersection Types?

Use | (union) and & (intersection) operators to combine multiple types.

type Admin = {
 role: "admin";
 permissions: string[];
};

type User = {
 name: string;
};

type AdminUser = Admin & User;

Note:
With union types, you can only access common fields; intersection types merge all fields.

9. Most Common TypeScript Mistakes

• Overusing any type
• Writing most code as plain JavaScript
• Poorly planned type hierarchies
• Using unnecessarily complex generics

Tip:
Maintain high type quality using tools like TSLint or ESLint.

10. How to Write Tests with TypeScript?

TypeScript integrates easily with testing libraries like Jest and Mocha. Type checking enables safer test scenarios.

test("add function works correctly", () => {
 expect(add(2, 3)).toBe(5);
});

Benefit:
Errors in tests can be caught at compile time.

To succeed in TypeScript interviews, it’s not enough to just know the syntax. You must understand how TypeScript differs from JavaScript, actively use the type system, learn integrations with React and Angular, and gain experience applying it to real-world problems.

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