Question 1

What is TypeScript and how does it differ from JavaScript?

Accepted Answer

TypeScript is a strongly-typed superset of JavaScript that compiles to plain JavaScript. It adds static type checking, interfaces, generics, and other features that catch errors at compile time rather than runtime. TypeScript improves code maintainability, IDE support (autocomplete, refactoring), and documentation. All valid JavaScript is valid TypeScript.

Question 2

What are the basic data types in TypeScript?

Accepted Answer

Primitive types: string, number, boolean, null, undefined, symbol, bigint. Special types: any (disables type checking), unknown (type-safe any), void (no return value), never (never returns). Object types: object, array (number[] or Array<number>), tuple ([string, number]), enum. TypeScript also supports literal types and custom types via interfaces and type aliases.

Question 3

What is type inference in TypeScript?

Accepted Answer

Type inference automatically determines types based on values without explicit annotations. TypeScript infers: variable types from initialization (let x = 5 infers number), function return types from return statements, and generic type parameters from arguments. Explicit types are optional when inference is clear. Inference reduces verbosity while maintaining type safety.

Question 4

What is the difference between interface and type alias?

Accepted Answer

Both define object shapes, but interfaces are extendable via 'extends' and declaration merging (same-named interfaces combine). Type aliases use '=' syntax and support unions, intersections, and mapped types more naturally. Interfaces are preferred for object shapes and class contracts; type aliases for unions, intersections, and complex types. Performance difference is negligible.

Question 5

What are union and intersection types?

Accepted Answer

Union types (A | B) allow a value to be one of several types—use type guards to narrow. Intersection types (A & B) combine multiple types into one with all properties from each. Union example: string | number accepts either. Intersection example: Person & Employee has all properties of both. Unions are 'or'; intersections are 'and'.

Question 6

What are generics in TypeScript?

Accepted Answer

Generics enable reusable components that work with multiple types while maintaining type safety. Define with angle brackets: function identity(arg: T): T. The type parameter T is determined at usage. Generics appear in functions, classes, and interfaces. Constraints () limit acceptable types. Common examples: Array, Promise, Map.

Question 7

What are access modifiers in TypeScript?

Accepted Answer

Access modifiers control member visibility in classes. public (default): accessible anywhere. private: accessible only within the declaring class. protected: accessible within the class and subclasses. readonly: can only be assigned in declaration or constructor. TypeScript also supports private fields using # prefix (true runtime privacy). Use for encapsulation and API design.

Question 8

What are type guards and how do you use them?

Accepted Answer

Type guards narrow types within conditional blocks. Built-in: typeof (primitives), instanceof (classes), 'in' operator (property existence), Array.isArray(). Custom type guards use 'is' keyword: function isString(x: unknown): x is string { return typeof x === 'string'; }. After the check, TypeScript knows the narrowed type. Essential for working with union types safely.

Question 9

What are type assertions in TypeScript?

Accepted Answer

Type assertions tell the compiler to treat a value as a specific type using 'as' syntax: value as Type, or angle bracket syntax: <Type>value (not in JSX). Use when you know more than TypeScript can infer. Unlike casting, assertions don't change runtime behavior—they're compile-time only. Prefer type guards when possible; assertions bypass type safety.

Question 10

What is the 'never' type?

Accepted Answer

The 'never' type represents values that never occur. Used for: functions that never return (throw errors or infinite loops), exhaustive type checking in switch statements (unreachable code after all cases handled), and impossible intersections. Never is assignable to all types but nothing is assignable to never. It enables compile-time exhaustiveness checks.

Question 11

What are decorators in TypeScript?

Accepted Answer

Decorators are special declarations (@ syntax) that modify classes, methods, properties, or parameters at design time. They're functions that receive the target and can add metadata or modify behavior. Types: class, method, accessor, property, parameter decorators. Common uses: dependency injection, logging, validation, ORM mapping. Require experimentalDecorators compiler option.

Question 12

How do you handle asynchronous operations in TypeScript?

Accepted Answer

Use Promises and async/await with proper typing. Promise represents eventual T value. Async functions return Promise. Type async function returns explicitly or let inference work. Handle errors with try/catch in async functions or .catch() on promises. TypeScript ensures you handle promise results correctly and catches type mismatches in async code.

Question 13

What are conditional types?

Accepted Answer

Conditional types select types based on conditions: T extends U ? X : Y. If T is assignable to U, result is X; otherwise Y. Enable powerful type transformations. Built-in utility types use them: Exclude, Extract, NonNullable. Combined with 'infer' keyword, extract types from other types. Essential for advanced generic programming and type manipulation.

Question 14

What are modules in TypeScript?

Accepted Answer

Modules are files with import/export statements, providing code organization and encapsulation. Export with 'export' keyword; import with 'import'. Supports named exports, default exports, re-exports, and namespace imports. Module resolution (node, classic) determines how imports are located. ES modules are standard; CommonJS for Node.js compatibility. Each file is its own scope.

Question 15

What are utility types in TypeScript?

Accepted Answer

Built-in utility types transform existing types. Partial: all properties optional. Required: all properties required. Readonly: all properties readonly. Pick: subset of properties. Omit: exclude properties. Record: object type with keys K and values T. ReturnType: function return type. Parameters: function parameter types. They reduce boilerplate significantly.

Data Types and Variables(8 questions)

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

What are the basic data types in TypeScript?

How do you declare a variable in TypeScript?

What is type inference in TypeScript?

How do you define a function in TypeScript?

What are optional parameters and how do you define them?

How do you define a function type in TypeScript?

What are generics in TypeScript and how do you use them in functions?

Interfaces and Classes in TypeScript(5 questions)

What is an interface in TypeScript?

What is a class in TypeScript?

Explain the difference between an interface and a class

How do you implement an interface in a class?

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

Advanced Types in TypeScript(6 questions)

What are union types and how do you use them?

What are intersection types and how do you use them?

What are type aliases and how do you use them?

What are conditional types and how do you use them?

Explain the concept of the "never" type in TypeScript

Key Takeaways

Object-Oriented Programming Interview Questions(4 questions)

What are the principles of object-oriented programming?

How are these principles implemented in TypeScript?

What is inheritance in TypeScript?

What is polymorphism in TypeScript?

Modules and Namespaces(4 questions)

What are modules in TypeScript?

What are namespaces in TypeScript?

Explain the difference between modules and namespaces.

How do you import and export modules in TypeScript?

Decorators(4 questions)

What are decorators in TypeScript?

How do you define a decorator?

What are some common use cases for decorators?

Additional Resources

Asynchronous Programming(3 questions)

How do you handle asynchronous operations in TypeScript?

What are promises and how do you use them?

What are async/await and how do you use them?

Type Guards and Type Assertions(3 questions)

What are type guards in TypeScript and when would you use them?

What are type assertions in TypeScript and when should you use them?

What's the difference between type guards and type assertions?

Advanced Generics(1 questions)

Explain how to use generics with interfaces and classes.

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

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

What are `async`/`await` and how do you use them?