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TypeScript Interview Questions

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Q1
What is TypeScript and how does it differ from JavaScript?

TypeScript is a superset of JavaScript that adds optional static typing, classes, interfaces, and modern ES features.

It compiles down to plain JavaScript and is fully compatible with existing JS code.

TypeScript enforces type checking at compile time, catching errors earlier in the development cycle.

It is widely used in large-scale applications, Angular, and React projects to improve maintainability and tooling support.

TypeScript brings structure, IntelliSense, and refactoring capabilities to JavaScript development.

Q2
What are types in TypeScript and why are they important?

TypeScript introduces static typing, allowing developers to declare variable, function, and object types at compile time.

Types include primitives, arrays, tuples, enums, any, unknown, void, never, null, and undefined.

Optional and readonly modifiers further refine type definitions.

Strong typing improves code reliability, documentation, and developer experience with autocompletion and refactoring.

Using types helps prevent common bugs and improves code maintainability in large-scale applications.

Q3
What are interfaces in TypeScript?

Interfaces in TypeScript define the shape of an object and are used for type-checking.

They are used to describe object structure, function signatures, and contract-based programming.

Interfaces support inheritance and can be extended or implemented by classes.

They are ideal for defining contracts and ensuring consistent APIs across modules and components.

Interfaces improve modularity, testability, and maintainability in large-scale TypeScript applications.

Q4
What is the difference between interface and type alias in TypeScript?
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Interfaces are used to define object shapes and are open-ended — they can be extended and merged.

Type aliases can name any type, including primitives, unions, intersections, and tuples.

Interfaces support inheritance and declaration merging, while type aliases offer more flexibility in type composition.

Use interfaces for object types and classes, and type aliases for union types, mapped types, and conditional types.

Choosing the right construct improves code organization and maintainability.

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