1. What is TypeScript?

TypeScript is a strongly typed programming language that builds on JavaScript, giving you better tooling at any scale. It's an open-source language developed by Microsoft that adds optional static typing and other features to JavaScript.

2. What is the difference between TypeScript and JavaScript?

TypeScript: A superset of JavaScript, supports static typing, and is compiled into JavaScript.
JavaScript: A dynamically typed scripting language used to create interactive web content.

3. What is the "Type System" in TypeScript?

A set of rules that assign types to variables, functions, and other entities in the code, allowing the compiler to catch errors like using a number where a string is expected.

4. What is the purpose of the `tsconfig.json` file?

The configuration file for a TypeScript project. It specifies the root files and compiler options required to compile the project.

5. What are the basic types in TypeScript?

string, number, boolean, array, tuple, enum, any, void, null, undefined, never, and object.

6. What is the difference between an `interface` and a `type` alias?

Interface: Can be extended and merged; preferred for defining object structures and API contracts.
Type: Can represent any type (including unions/intersections); cannot be merged; preferred for complex type definitions.

7. What is "Any" type?

The any type allows a variable to hold any value, effectively disabling type checking for that variable. It should be used sparingly as it negates the benefits of TypeScript.

8. What is "Unknown" type?

A safer alternative to any. You can't perform any operations on an unknown value without first checking its type (type narrowing).

9. What is "Never" type?

Represents the type of values that never occur. Commonly used for functions that always throw an exception or never return (e.g., infinite loops).

10. What are "Unions" and "Intersections"?

Union (|): Allows a variable to hold values of multiple types (e.g., string | number).
Intersection (&): Combines multiple types into one (e.g., TypeA & TypeB).

11. What is "Type Inference"?

A feature where the TypeScript compiler automatically determines the type of a variable based on the value assigned to it.

12. What is "Type Casting" (Type Assertion)?

A way to tell the compiler that you know more about the type of a value than it does (using <Type> or as Type).

13. What is "Type Guarding" (Type Narrowing)?

A technique to refine the type of a variable within a specific conditional block (using typeof, instanceof, or custom type predicates).

14. What are "Generics"?

Generics allow you to create reusable components that can work with a variety of types while still maintaining type safety (e.g., Array<T>).

15. What is an "Enum"?

A way to define a set of named constants, either numeric or string-based.

16. What is the difference between `null` and `undefined` in TypeScript?

Both represent the absence of a value, but null is usually an intentional absence, while undefined means a value hasn't been assigned yet. In TypeScript, they are separate types unless strictNullChecks is disabled.

17. What is a "Tuple"?

An array with a fixed number of elements whose types are known but not necessarily the same (e.g., [string, number]).

18. What are "Access Modifiers"?

Keywords used to control the visibility of class members:

public: Accessible from anywhere.
private: Accessible only within the class.
protected: Accessible within the class and its subclasses.

19. What is the purpose of the `readonly` modifier?

It makes a property immutable once it has been initialized.

20. What is "Abstract Class"?

A base class that cannot be instantiated directly. It's meant to be extended by other classes and may contain abstract methods that must be implemented by subclasses.

21. What is "Static" keyword?

Used to define properties and methods that belong to the class itself, rather than to instances of the class.

22. What are "Decorators"?

A special kind of declaration that can be attached to a class, method, accessor, property, or parameter. They are a feature of ES2015+ and are used to "annotate" and "modify" the code.

23. What is "Namespace"?

A way to organize code into logical groups, preventing name collisions in the global scope. (ES Modules are preferred over namespaces in modern development).

24. What is "Declaration Merging"?

A feature where multiple declarations with the same name are merged into a single definition (e.g., merging two interfaces with the same name).

25. What are "Ambient Declarations" (`.d.ts` files)?

Files that provide type information for JavaScript libraries that don't have their own TypeScript types.

26. What is the "Record" utility type?

Used to define an object type with specific keys and values (e.g., Record<string, number>).

27. What is the "Partial" utility type?

Makes all properties of an object type optional.

28. What is the "Required" utility type?

Makes all properties of an object type required (removing the ? modifiers).

29. What is the "Readonly" utility type?

Makes all properties of an object type read-only.

30. What is the "Pick" utility type?

Creates a new type by choosing a specific subset of properties from an existing type.

31. What is the "Omit" utility type?

Creates a new type by excluding a specific subset of properties from an existing type.

32. What is the "Exclude" and "Extract" utility type?

Exclude: Removes certain types from a union.
Extract: Selects certain types from a union.

33. What is the "NonNullable" utility type?

Removes null and undefined from a union type.

34. What is "Optional Chaining" (`?.`)?

Allows you to safely access nested properties of an object even if an intermediate property is null or undefined.

35. What is the "Nullish Coalescing" operator (`??`)?

Returns the right-hand operand only if the left-hand operand is null or undefined.

36. What is "Keyof" operator?

Returns a union of the keys of an object type.

37. What is "Typeof" operator?

Used to extract the type of a variable or property.

38. What is "Indexed Access Type"?

Allows you to look up the type of a specific property on another type (e.g., User["id"]).

39. What is "Mapped Type"?

Allows you to create a new type based on an old one by transforming its properties (e.g., turning all properties to boolean).

40. What is "Conditional Type"?

A type that depends on a condition (e.g., T extends string ? "A" : "B").

41. What is "Template Literal Type"?

Allows you to build new string types by manipulating existing ones (e.g., `${Brand}-${Model}`).

42. What is "Function Overloading"?

A way to define multiple function signatures for a single function, allowing it to handle different types of inputs and return different types.

43. What is "ThisType" utility?

Used to specify the type of this within an object literal.

44. What is the difference between `implements` and `extends`?

extends: Inherits from a class or interface.
implements: Tells a class to adhere to the structure of an interface.

45. What is "Strict Type Checking"?

A set of compiler options (like strictNullChecks, noImplicitAny) that make TypeScript's type enforcement more rigorous.

46. What is "Nominal Typing" vs "Structural Typing"?

Structural: (TypeScript) Two types are compatible if they have the same structure.
Nominal: Two types are compatible only if they have the same name.

47. What is "Mangled Type"?

(Not a standard TS term, but often refers to type obfuscation). TypeScript doesn't mangle types during compilation; it just removes them.

48. What is the "ConstructorParameters" and "InstanceType" utility?

ConstructorParameters: Extracts the tuple of parameters for a constructor function.
InstanceType: Extracts the type of an instance of a class.

49. What is "Type Predicate" (`arg is Type`)?

A special return type for a function that serves as a custom type guard.

50. What is a "Literal Type"?

A type that represents a specific value (e.g., type Direction = "North" | "South" | "East" | "West").

TypeScript Interview Questions

Table of Contents

What is TypeScript?

What is the difference between TypeScript and JavaScript?

What is the "Type System" in TypeScript?

What is the purpose of the <code>tsconfig.json</code> file?

What are the basic types in TypeScript?

What is the difference between an <code>interface</code> and a <code>type</code> alias?

What is "Any" type?

What is "Unknown" type?

What is "Never" type?

What are "Unions" and "Intersections"?

What is "Type Inference"?

What is "Type Casting" (Type Assertion)?

What is "Type Guarding" (Type Narrowing)?

What are "Generics"?

What is an "Enum"?

What is the difference between <code>null</code> and <code>undefined</code> in TypeScript?

What is a "Tuple"?

What are "Access Modifiers"?

What is the purpose of the <code>readonly</code> modifier?

What is "Abstract Class"?

What is "Static" keyword?

What are "Decorators"?

What is "Namespace"?

What is "Declaration Merging"?

What are "Ambient Declarations" (<code>.d.ts</code> files)?

What is the "Record" utility type?

What is the "Partial" utility type?

What is the "Required" utility type?

What is the "Readonly" utility type?

What is the "Pick" utility type?

What is the "Omit" utility type?

What is the "Exclude" and "Extract" utility type?

What is the "NonNullable" utility type?

What is "Optional Chaining" (<code>?.</code>)?

What is the "Nullish Coalescing" operator (<code>??</code>)?

What is "Keyof" operator?

What is "Typeof" operator?

What is "Indexed Access Type"?

What is "Mapped Type"?

What is "Conditional Type"?

What is "Template Literal Type"?

What is "Function Overloading"?

What is "ThisType" utility?

What is the difference between <code>implements</code> and <code>extends</code>?

What is "Strict Type Checking"?

What is "Nominal Typing" vs "Structural Typing"?

What is "Mangled Type"?

What is the "ConstructorParameters" and "InstanceType" utility?

What is "Type Predicate" (<code>arg is Type</code>)?

What is a "Literal Type"?