Top 70+ TypeScript Interview Questions and Answers [2025]

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These TypeScript interview questions cover fundamental concepts such as data types, variable declarations, functions, interfaces, and type annotations. They are ideal for entry-level interviews or for those new to the language, forming a solid base before exploring advanced features.

1. What Is TypeScript?

TypeScript, an open-source programming language created by Microsoft, is a syntactical superset of JavaScript that adds optional static typing and object-oriented programming features. All valid JavaScript code is also valid TypeScript code. Optional type annotations help improve code quality and maintenance.

It’s code is transpiled into regular JavaScript code, allowing it to run in any environment where JavaScript runs, such as browsers or Node.js. In the context of TypeScript interview questions, this forms the foundational concept every candidate should know.

2. What Are the Key Features of TypeScript?

TypeScript strengthens JavaScript with several features that improve the development experience and code quality:

• Static Typing: Type checking is performed during development, allowing errors to be detected early.
• Advanced Tooling: Includes IntelliSense, autocompletion, and safe refactoring.
• Object-Oriented Programming Features: Supports classes and interfaces for structured, reusable code.
• JavaScript Interoperability: Can be compiled to plain JavaScript, enabling use with existing libraries and frameworks.
• Early Error Detection: Highlights errors during the early development stage.

These features are frequently discussed in TypeScript interview questions due to their importance in scalable application development.

3. What Are the Primitive Data Types in TypeScript?

In TypeScript, the primary primitive data types are:

• string: Represents textual data, e.g., "hello" or "TypeScript".
• number: Represents both integers and floating-point numbers, e.g., 42 or 3.14.
• boolean: Represents logical values: true or false.

Understanding these is a common requirement in TypeScript interview questions.

4. What Are the Advantages of Using TypeScript Over JavaScript?

Advantages include:

• Static Typing: Detects errors early, making code more reliable.
• Better Maintainability: Explicit types make code easier to read and refactor.
• Enhanced Development Experience: Provides better IntelliSense, debugging, and error messaging.
• OOP Support: Includes interfaces, generics, and classes.
• JavaScript Compatibility: Works seamlessly with JavaScript and can be introduced incrementally.
• Strong Tooling Support: Integrates with frameworks like React, Angular, and Vue.
• Improved Scalability: Type checking supports large-scale applications.

These points often appear in TypeScript interview questions to assess practical knowledge of the language’s benefits.

5. List the Applications of TypeScript

Applications include:

• Client-side development for web applications
• Server-side development using Node.js
• Developing large-scale JavaScript applications
• Building enterprise-level apps with maintainable code
• Framework-based development, especially with Angular
• Developing reusable components for frontend projects
• Mobile application development using frameworks like Ionic

These use cases are often referenced in TypeScript interview questions to assess where the language fits in real-world projects.

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6. Explain How Arrays Work in TypeScript

In TypeScript, arrays store multiple values of the same type. You can define an array as:

• let arrayName: type[] = [values];
• let arrayName: Array<type> = [values];

Arrays only accept elements of the specified type. Adding a value of a different type causes a compile-time error.

Arrays also support methods like push(), pop(), concat(), and iteration methods such as for, forEach, and for...of. Understanding this is essential for many TypeScript interview questions.

7. What Is Type Inference in TypeScript?

Type inference means TypeScript automatically assigns a type to a variable if no explicit type is given.

Example:

let message = "Hello";

Here, TypeScript infers the message as a string. This reduces the need for manual type annotations while keeping type safety. Type inference is a core topic in TypeScript interview questions.

8. What Is the ‘Any’ Type, and When Should It Be Used?

The any type is used when a variable’s type is unknown at the time of writing code. It disables type checking for that variable, allowing values of any type. It’s useful for migrating JavaScript code to TypeScript or handling dynamic content from APIs.

However, it should be used sparingly, as it removes TypeScript’s type safety. This is a frequent point in TypeScript interview questions regarding type safety trade-offs.

9. What Is ‘Void’, and When Should the ‘Void’ Type Be Used?

The void type represents the absence of a return value in a function.

Example:

function greet(): void {
  console.log("Hello");
}

It’s commonly used for functions that perform actions without returning data. Questions about void are standard in TypeScript interview questions.

10. What Is the ‘Unknown’ Type in TypeScript?

The unknown type can hold any value, but it is safer than any because type checks must be done before use. It’s used when the variable’s type is unknown but still requires type safety. This concept appears in TypeScript interview questions related to safe type handling.

11. What Is the ‘Never’ Type in TypeScript?

The never type represents values that never occur, used for functions that never return (e.g., throw errors or run infinitely). Variables of type never can hold any value. The never type is often part of advanced TypeScript interview questions.

12. What Are Tuples in TypeScript?

A tuple is a fixed-length array where elements can have different types. Tuples are useful for representing data structures with a known number and type of elements. This concept regularly appears in TypeScript interview questions to test type control understanding.

13. What Is an Enum in TypeScript?

Enums are used to define a set of named constants, either numeric or string-based. They make code more readable and maintainable when working with related values. Enums are a standard topic in TypeScript interview questions.

14. How Are Functions Defined in TypeScript?

Functions can include optional type annotations for parameters and return values. This ensures correct types at compile time, improving readability and maintainability. Knowing function definitions is necessary for TypeScript interview questions.

15. What Are Optional Parameters in TypeScript?

Optional parameters, marked with ?, allow a function to be called without certain arguments. They must come after required parameters and default to undefined if not provided. This is a common TypeScript interview questions topic.

16. What Are Default Parameters in TypeScript?

Default parameters allow function arguments to have preset values if no argument is provided. This improves flexibility and removes the need for extra checks inside the function body. Understanding default parameters is part of many TypeScript interview questions.

17. What Are Rest Parameters in TypeScript?

Rest parameters, defined with ..., allow a function to accept an indefinite number of arguments as an array. They are useful when the number of inputs is unknown or variable. Rest parameters often appear in TypeScript interview questions about function flexibility.

18. What Is the Difference Between ‘Let’, ‘Var’, and ‘Const’?

In TypeScript:

• var: Function-scoped, can be re-declared and updated.
• let: Block-scoped, can be updated but not re-declared in the same scope.
• const: Block-scoped, must be initialized and cannot be updated or re-declared.

This distinction is common in TypeScript interview questions focused on variable scoping and best practices.

19. What Is an Interface in TypeScript?

An interface defines the structure an object must follow, including property names, types, and method signatures. Interfaces help enforce type consistency. They are frequently included in TypeScript interview questions.

20. How Do You Implement Interfaces in Classes?

Classes use the implements keyword to follow the structure defined by an interface. If a class fails to implement all members, a compile-time error occurs. This is a standard TypeScript interview questions topic for OOP concepts.

21. What Is the Difference Between Overloading and Overriding?

Interfaces define the shape of objects and can be extended, while types can represent unions, intersections, and other complex types. Interfaces are mainly used for object contracts, types are more flexible.

This comparison is often asked in TypeScript interview questions to test knowledge of type system flexibility.

22. Can Interfaces Be Extended in TypeScript?

Yes. Interfaces can extend one or more interfaces using extends, inheriting their members. This is a common point in TypeScript interview questions on reusability and structure.

23. What Are Access Modifiers in TypeScript?

In TypeScript, access modifiers are used to control the visibility and accessibility of class members (properties and methods). There are three main access modifiers:

• public: Accessible from anywhere (default).
• private: Accessible only within the class.
• protected: Accessible in the class and subclasses.

Access modifiers are standard in TypeScript and have been often asked in many of the TypeScript interview questions about encapsulation.

24. What Is the ‘Readonly’ Modifier in TypeScript?

readonly makes a property immutable after initialization. It helps protect constants in a class. This is often mentioned in TypeScript interview questions about immutability. readonly helps prevent accidental updates to values that should stay constant throughout the life of an object, adding an extra layer of safety to your code.

25. What Is a Namespace in TypeScript?

A namespace groups related code (interfaces, classes, functions, variables) under a single name to avoid conflicts. While ES6 modules are now preferred, namespaces still appear in TypeScript interview questions about legacy code organization.

26. What Is a Module in TypeScript?

A module is any file with its own scope that uses export and import to share code. Modules help organize and reuse code. They are frequently included in TypeScript interview questions.

27. How Do You Import and Export Modules in TypeScript?

In TypeScript, the export keyword is used to make classes, functions, variables, or interfaces available outside the current file. The import keyword is then used to bring those exported members into another file.

For example:

• To export: export class Person
• To import: import { Person } from './Person'

For example:

• export default class Logger
• import Logger from './Logger'

This approach supports modular development and helps maintain organized, reusable code.

28. What Are Type Aliases in TypeScript?

Type aliases give custom names to types (primitive, union, intersection, object, etc.) using the type keyword. They improve readability and are part of many TypeScript interview questions.

29. What Is Type Assertion in TypeScript?

Type assertion tells the compiler to treat a value as a specific type without changing runtime behavior.

Syntax:

• Angle-bracket syntax: <Type> value
• as syntax: value as Type

Type assertion is a common TypeScript interview questions topic, especially with DOM or JSON data.

30. How Do You Define Optional Properties in Interfaces?

In TypeScript, optional properties in interfaces are defined by appending a question mark (?) to the property name. This indicates that the property is not mandatory when creating objects that implement the interface

For example:

interface User {
  name: string;
  age?: number;
}

In this User interface, the name property is required, while the age property is optional. This means that objects of type User can either include or omit the age property.

This feature enhances flexibility in defining object structures where certain properties may not always be applicable or available.

31. What Are Index Signatures in TypeScript?

Index signatures allow defining objects with dynamic property keys of a specific type. They’re useful when the shape of an object isn't known in advance, but all property keys and values follow a consistent type pattern.

The set of TypeScript interview questions helps you go beyond basic concepts. These TypeScript interview questions cover generics, decorators, type guards, utility types, and more.

32. How Is a Union Type Used in TypeScript?

A union type in TypeScript is employed to enable a variable to take more than one type. It is denoted by using the pipe (|) symbol between types. This provides more freedom during the assignment of values without losing the advantage of TypeScript's type checking.

For example:

let value: string | number; 

A union type in TypeScript is employed to enable a variable to take more than one type. It is denoted by using the pipe (|) symbol between types. This provides more freedom during the assignment of values without losing the advantage of TypeScript's type checking.

33. What Is an Intersection Type in TypeScript?

An intersection type in TypeScript combines multiple types into one. It is represented using the ampersand (&) symbol. A value of an intersection type must satisfy all the combined types.

For example:

type Person = { name: string };
type Employee = { id: number };
type Staff = Person & Employee;

In this case, the Staff must have both name and ID properties. Intersection types are useful when merging multiple type definitions into a single, more comprehensive one.

34. Define Literal Types in TypeScript

Literal types in TypeScript are defined using specific values as types, rather than general types like string or number. These can include string, number, or boolean literals. Literal types are used to restrict a variable to a fixed set of values.

type Direction = "up" | "down";

35. What Are Template Literal Types?

Template literal types in TypeScript allow for the construction of new string types by combining string literals and placeholders. They work similarly to template strings in JavaScript but are used at the type level.

For example:

type Greeting = Hello, {"{"$string"}"};
let message: Greeting;
message = "Hello, John";  // valid
message = "Hi, John";     // error

Template literal types enable more dynamic and flexible type definitions, especially useful when creating patterns based on string values.

36. What Is the Significance of Triple-Slash Directives?

Triple-slash directives in TypeScript are unique comments that are employed to give directions to the TypeScript compiler. They are mainly employed for referencing other files, including type declaration files, and also to determine the way the compiler handles those files.

For example:

/// <reference path="myFile.d.ts" />

The key significance of triple-slash directives is to:

• Include external files in the compilation process.
• Link additional type declaration files.
• Enable type checking and better intellisense support for external libraries.

They are crucial when dealing with projects that include multiple files or libraries, especially in a modular codebase.

37. What Is JSX in TypeScript?

JSX in TypeScript allows you to write HTML-like code within your TypeScript files. It's most commonly used with React to construct user interfaces. It's a means of describing what the UI is supposed to look like, but done in an easy-to-read and write syntax.

The great thing about TypeScript is that it validates your JSX code for errors as you progress, so you have the advantage of both type safety and the flexibility of JSX. Files with JSX in TypeScript are saved using the .tsx extension, indicating to TypeScript that this file includes JSX.

38. How Do You Create a Generic Function?

A generic function is defined by introducing a type parameter within angle brackets () prior to the parameter list of a function. It enables a function to take parameters of different types while maintaining type safety.

For example:

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

Here, T is a placeholder for any type that will be passed when the function is called. This makes the function reusable for multiple data types without losing type information.

39. How Are Generic Classes Used?

Generic classes in TypeScript allow defining class templates that work with different data types. A type parameter (like ) is declared with the class name and used throughout the class for properties, methods, or constructor parameters. This makes the class flexible and type-safe across various data types.

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

When creating an object of the class, a specific type is passed:

const numberBox = new Box<number>(123);
const stringBox = new Box<string>("Hello");

This way, the class maintains strict typing while being reusable across types.

40. How Do You Create a Generic Interface?

You can define a generic interface in TypeScript using a type parameter in angle brackets () and placing this after the name of the interface. TypeScript will allow the interface to operate with several different data types, but will maintain type safety in that it will track what the type is throughout the interaction.

interface Container<T> {
  value: T;
  getValue: () => T;
}

When using the interface, a specific type is provided:
const stringContainer: Container<string> = {
  value: "Hello",
  getValue: () => "Hello",
};

This approach ensures that all properties and methods within the interface consistently use the defined type.

41. What Is a Constraint in Generics?

In TypeScript, a constraint in generics restricts the types that can be used as arguments for a generic type. This is done using the extends keyword. Constraints ensure that the generic type adheres to a specific structure or type, providing more control and preventing runtime errors.

interface Lengthwise {
  length: number;
}

function logLength<T extends Lengthwise>(item: T): void {
  console.log(item.length);
}

Here, T is constrained to types that have a length property. This means the function logLength can only be used with values like arrays, strings, or objects that define length.

42. State the Difference Between ‘Interface’ and ‘Abstract Class’

In TypeScript, both interfaces and abstract classes are used to define the structure that other classes can implement or extend. An interface defines a contract or a shape for an object. It cannot contain any implementation, only method and property declarations.

An abstract class, on the other hand, is a partially implemented class. It can have both implemented methods and abstract methods that must be overridden in derived classes.

43. How Do You Define and Use Decorators in TypeScript?

Decorators in TypeScript are special declarations used to modify the behavior of classes, methods, properties, or parameters. They are prefixed with @ and require the experimentalDecorators flag to be enabled in tsconfig.json. There are several types of decorators:

• Class decorators: Applied to classes.
• Method decorators: Applied to methods.
• Property decorators: Applied to properties.
• Parameter decorators: Applied to constructor or method parameters.

To use decorators:

• Enable "experimentalDecorators": true in tsconfig.json.
• Define a decorator function that receives metadata (like the target, property name, or descriptor).
• Apply it using @DecoratorName syntax above the class/member.

44. What Is the Purpose of Experimental Decorators?

The intention behind experimental decorators in TypeScript is to enable developers to add special annotations or behavior to classes, methods, accessors, properties, or parameters. They are a stage 2 proposal for JavaScript and offer a very potent method of writing reusable, declarative code, particularly in frameworks such as Angular.

They are "experimental" because the feature hasn't been included in the official JavaScript standard yet. To utilize them, experimentalDecorators needs to be set to true in the tsconfig.json file. After that is enabled, decorators can be used to add metadata, do dependency injection, log method calls, or change how a class works at runtime.

45. Explain the Working of Enums in TypeScript?

In TypeScript, enums provide a way to define a set of named constants, which can either be numeric or string values. They are used to represent a collection of related values in a more readable and type-safe manner. Here's how enums work in TypeScript:

• Numeric Enums: By default, enums in TypeScript are numeric. The first member of an enum is assigned the value 0, and subsequent members are automatically incremented by 1 unless explicitly assigned a value. You can also explicitly assign numeric values to specific enum members.
• String Enums: In string enums, each member of the enum is assigned a string value explicitly. Unlike numeric enums, string enums do not auto-increment.
• Heterogeneous Enums: TypeScript allows you to mix numeric and string values in an enum, creating what’s known as a heterogeneous enum. This is less common but can be used in certain situations.
• Const Enums: When performance is critical, const enums can be used. These enums are not evaluated at runtime but instead are inlined at compile time. This means no actual enum object is generated, making the code more efficient.
• Reverse Mapping for Numeric Enums: TypeScript provides reverse mapping for numeric enums, meaning you can access an enum member by its value. This feature works only for numeric enums.

46. What Is Type Narrowing in TypeScript, and How Does It Work?

Type narrowing is the process of refining the type of a variable within a specific scope, typically through conditional checks. TypeScript uses control flow analysis to determine the type of a variable after these checks.

interface Lengthwise {
  length: number;
}

function logLength<T extends Lengthwise>(item: T): void {
  console.log(item.length);
}

In this example, the typeof check narrows the type of id to string within the if block and to number within the else block.

47. How Can You Ensure Exhaustive Checks In TypeScript Using the Never Type?

TypeScript uses the never type to enforce exhaustive checks in a switch statement, especially during discriminated union type evaluations. Assigning the variable to never in the default switch condition enables TypeScript to detect new union variants that remain unhandled during compilation time. This ensures that all cases are explicitly covered.

type Shape = { kind: 'circle'; radius: number } | { kind: 'square'; sideLength: number };

function getArea(shape: Shape): number {
  switch (shape.kind) {
    case 'circle':
      return Math.PI * shape.radius ** 2;
    case 'square':
      return shape.sideLength ** 2;
    default:
      const _exhaustive: never = shape; // Compile-time check
      return _exhaustive;
  }
}

TThis method guarantees that every variant of the union type is accounted for and prevents unhandled cases from slipping through.

48. What Are Ambient Modules in TypeScript and When Would They Be Used?

They are used to declare modules for external JavaScript libraries or global variables that TypeScript doesn’t have direct access to. They are defined using the declare module syntax in .d.ts files, providing type information for code not written in TypeScript.

declare module 'some-external-library' {
  export function someFunction(): void;
}

When to use:

• When integrating third-party libraries without TypeScript definitions.
• For global variables or scripts included in the environment.
• To ensure type safety for external JavaScript code.

49. How Can Modules Be Re-exported in TypeScript, and What Are the Best Practices?

Modules in TypeScript can be re-exported with the export keyword for code modularity and organization. This method provides the possibility to aggregate exports of several modules into one module to make imports easy for consumers.

Best practices:

• Avoid export * from for re-exporting: Using export * from can inadvertently export unintended symbols, leading to namespace pollution and potential conflicts. It's recommended to explicitly specify the exports to re-export.
• Use barrel files judiciously: Barrel files (e.g., index.ts) can simplify imports by aggregating exports from multiple modules. However, overuse can lead to circular dependencies. It's advisable to use barrel files carefully and avoid them when they introduce complexity.
• Re-export types with export type: When re-exporting types, especially with the --isolatedModules flag enabled, always use the export type syntax to ensure clarity and prevent errors.

50. What Is the Use of the tsconfig.json File in TypeScript?

The tsconfig.json file configures TypeScript projects by specifying which files are included and defining compiler options. It ensures consistent compilation settings across environments, enables features like autocompletion and error checking, and facilitates integration with build tools.

It is essential for managing large projects and supports incremental compilation with project references.

51. What Are the Key Options in tsconfig.json?

Some important options in tsconfig.json include:

• compilerOptions: Controls how the code is compiled (e.g., target, module, strict, outDir).
• include: Specifies which files or directories to include in the compilation.
• exclude: Lists files or folders to be excluded.
• files: Lists specific files to compile.
• lib: Specifies built-in library files to include (like ES6, DOM). These settings help customize the TypeScript compilation process.

52. State the Difference Between Internal Modules and External Modules in TypeScript

In TypeScript, the terms "internal modules" and "external modules" have evolved over time. Previously, "internal modules" referred to namespaces, and "external modules" referred to ES modules.

As of TypeScript 1.5, the terminology has been updated to align with ECMAScript 2015 standards:

• Namespaces (formerly "internal modules"): Provide a way to organize code within a single global scope.
• Modules (formerly "external modules"): Allow code to be split across multiple files, each with its own scope, and support importing/exporting functionality between them.

53. Explain TypeScript Definition Manager and Its Use

TypeScript Definition Manager (TSD) was a CLI tool used to manage .d.ts files, which provide type definitions for JavaScript libraries. It enabled developers to use external libraries with full type support, improving static type checking and reducing runtime errors.

TSD has now been deprecated and replaced by the @types system, which uses the DefinitelyTyped repository. Developers now install type definitions via npm using @types scoped packages, such as @types/jquery, simplifying type management in modern TypeScript projects.

54. Explain Recursive Type Aliases in TypeScript

A recursive type alias in TypeScript is a type that references itself in its definition. Recursive type aliases are effective for modeling nested or hierarchical structures like trees, JSON, or linked list data structures.

A type definition can sometimes reference itself through its properties, or directly (the alias name uses itself as a type), which allows TypeScript to effectively describe very complex and repeating self-referencing data structures.

However, you must be careful to ensure recursion is properly structured to avoid infinite loops in type resolution.

55. What Is Declaration Merging in TypeScript?

Declaration merging in TypeScript happens when the compiler combines multiple declarations with the same name into a single definition. This is commonly used with interfaces, namespaces, and function overloads.

interface Person {
  name: string;
}
interface Person {
  age: number;
}
// Merged: { name: string; age: number; }

It's helpful for type extension, particularly when dealing with third-party libraries or adding to existing types.

56. How Do You Install TypeScript Definitions Using DefinitelyTyped?

DefinitelyTyped TypeScript definitions are installed via npm with the @types scope. Like - to install types for a library such as lodash, the npm install --save-dev @types/lodash command is used.

This gives TypeScript the required.d.ts files to know about the types of the library without altering the original library.

57. What Is Module Resolution in TypeScript?

Module resolution is the process TypeScript uses to locate and link imported modules. It determines how an import path is mapped to a file on disk. TypeScript supports two strategies: Classic (older, non-node-style) and Node (mirrors Node.js behavior). Configuration options like baseUrl, paths, and moduleResolution in tsconfig.json influence this process.

58. State the Difference Between CommonJS and ES6 Modules

CommonJS uses require() and module.exports for importing and exporting modules, and it works synchronously, making it ideal for server-side applications like Node.js. In contrast, ES6 modules use import and export, supporting asynchronous loading, which is more efficient for modern JavaScript, especially in browsers.

ES6 modules are statically analyzed, making it easier for bundlers to optimize the code. While CommonJS is mainly used in Node.js, ES6 modules are now the standard for both client-side and server-side JavaScript.

59. What Is Type Compatibility?

Type compatibility in TypeScript has to do with how types relate to and can be assigned to one another. Compatibility establishes whether a value of a given type can be assigned to a variable of another type. TypeScript utilizes structural typing: two types are compatible if their structures are the same (that is, properties and methods to type), regardless of whether their names are the same at all.

For example, if two types have the same shape, they are both compatible even if they were not declared to be the same type. This opens up a large amount of flexibility for how types may relate to each other, and this relates to how dynamic and reusable projects can be in TypeScript.

60. What Is Structural Typing in TypeScript?

Structural typing is TypeScript’s type system model, where compatibility between types is determined by their actual structure or shape rather than their explicit names.

If two types have the same set of required properties and types, they are considered compatible. This allows for greater flexibility and is especially useful when working with objects from different sources or libraries, as long as their structures align.

This concept is often included in TypeScript interview questions, as it tests a developer’s understanding of TypeScript’s core type system behavior and how it differs from nominal typing in other languages.

61. What Are Mapped Types in TypeScript?

Mapped types in TypeScript allow the creation of new types by transforming existing ones. They iterate over the keys of a given type using the keyof operator and apply transformations to each property.

This is especially useful for creating utility types like Partial, Readonly, or custom modifications to all properties of a type, such as making them optional, readonly, or changing their value types. Mapped types enable powerful, reusable type manipulation patterns.

type Readonly<T> = {
  readonly [P in keyof T]: T[P];
};
interface Person {
  name: string;
  age: number;
}
type ReadonlyPerson = Readonly<Person>;
let person: ReadonlyPerson = {
  name: 'Alice',
  age: 30
};
// person.name = 'Bob'; // Error: Cannot assign to 'name' because it is a read-only property.

62. Explain the “As” Syntax in TypeScript

Type assertions in TypeScript are done using the as syntax. They provide a way to override the type that TypeScript inferred for a variable. If you know a value's type is more specific than what TypeScript inferred, you can tell the compiler that your knowledge of the type is more accurate.

let someValue: any = "this is a string";
// Using as syntax
let strLength: number = (someValue as string).length;

Type assertions with as are useful when TypeScript's type inference is not sufficient, or when working with code that has not been fully typed. This is a frequent topic in TypeScript interview questions, as it checks understanding of when and how to guide the compiler toward more precise type information.

63. What Is the ‘In’ Operator Used for?

The in operator in TypeScript allows you to check the presence of a given property and can also help in type guards in order to narrow types down, in particular, union types. This question often appears in TypeScript interview questions, as it tests a developer’s understanding of type narrowing and safe property checks within complex type structures.

TypeScript knows we have checked a property; it can better use inference to determine the actual type of an object. If we can always check for a property's presence, TypeScript can make our code more predictable and type-safe.

64. How Do You Use Discriminated Unions?

Discriminated unions in TypeScript provide a strong mechanism to handle a group of related types while keeping system safety. They work by having a common property that separates the possible shapes; this is called a "discriminant" or "tag." Let's see how they're used:

• Define the union type: Each variant has a unique type property that acts as a discriminator:



type Shape =
  | { type: "circle"; radius: number }
  | { type: "square"; sideLength: number }
  | { type: "rectangle"; width: number; height: number };



• Use a type guard to narrow the type: You can check the type property to safely access other properties:



function getArea(shape: Shape): number {
  switch (shape.type) {
    case "circle":
      return Math.PI * shape.radius ** 2;
    case "square":
      return shape.sideLength ** 2;
    case "rectangle":
      return shape.width * shape.height;
    default:
      throw new Error("Unknown shape");
  }
}

const myShape: Shape = { type: "circle", radius: 5 };
console.log(getArea(myShape)); // Outputs the area of the circle



• Ensure type safety with exhaustive checks: TypeScript will warn you if you forget to handle a case, making it great for maintainability.

Here, we will go through TypeScript advanced interview questions to help test deep technical understanding and problem-solving ability. These TypeScript interview questions are best suited for senior-level candidates.

65. What Is noImplicitAny in TypeScript?

noImplicitAny is a compiler option in the tsconfig.json file of a TypeScript project. It controls how the TypeScript compiler handles variables and function parameters that do not have an explicitly defined type.

By default, if TypeScript is unable to infer a type, it will implicitly assign the any type. This can cause possible runtime errors since any type turns off type checking for that variable. By setting noImplicitAny to true, it forces stricter type checking by preventing the compiler from automatically assigning any to unannounced types.

Behavior:

When noImplicitAny is set to false:

TypeScript allows implicit any types. For example, if you declare a function parameter without a type, it silently gets typed as any.

function log(message) {
  console.log(message); // message is implicitly of type 'any'
}

When noImplicitAny is set to true:

TypeScript will throw a compile-time error for variables or parameters that lack a type and cannot be inferred.

function log(message) {
  // Error: Parameter 'message' implicitly has an 'any' type.
  console.log(message);
}

66. Explain How Optional Chaining Works in TypeScript

Optional chaining in TypeScript facilitates safely getting deeply nested properties or methods without the need to manually check each level. Using the ? operator, it short-circuits and returns undefined if a reference is null or undefined instead of throwing a runtime error. It is particularly useful working with data from external sources such as APIs, where some of the properties do not exist. This results in cleaner and more readable code without unnecessary null checks.

67. What Is Void, and When to Use the Void Type in TypeScript?

In TypeScript, the void type signifies the absence of any value. It's generally employed as the return type of functions that do not return anything. It serves to indicate clearly the intent that a function is doing something as a side effect (such as logging or updating data) instead of calculating and returning some value. It's also applied in function types to indicate that a callback or handler is not anticipated to return anything.

68. What Is a Declaration File (.d.ts) and How Is It Used in TypeScript?

A .d.ts (declaration) file in TypeScript serves as a way to describe the shape of JavaScript code to the TypeScript compiler. Declaration files describe the type information present in existing JavaScript libraries, some of which may not have been written in TypeScript.

Declaration files describe the types, interfaces, functions, and variables without specifying the implementations, which allows JavaScript developers to reference third-party libraries with full type checking support and IntelliSense support.

This question often appears in many TypeScript interview questions, as declaration files in TypeScript are an important topic. They are especially useful when working with external libraries and can either be manually written or installed (or imported) from an existing project as an @types package on DefinitelyTyped.

69. How Do You Write a Custom .d.ts File?

To write a custom .d.ts file, create a new file with the .d.ts extension and use TypeScript's declare keyword to define the shape of external or global entities like modules, functions, variables, or classes.

It should not contain actual implementations, just type information. For example, when integrating a plain JavaScript library without type definitions, declare its types and export them so TypeScript can recognize and validate their usage throughout the project.

This is another topic that comes up often in TypeScript interview questions, as writing custom declaration files demonstrates your ability to work with non-TypeScript code, configure projects correctly, and ensure type safety across the application.

70. What Are the Advanced Types in TypeScript?

Advanced types in TypeScript are features that let you model more complex type behaviors and relationships beyond basic types like string, number, or boolean. They help make your code more flexible, expressive, and safer by better capturing how values relate and behave.

Here are some key categories of advanced types:

• Intersection Types (&): Combines multiple types into one. A value must satisfy all the types. Example: type AdminUser = User & Admin;
• Union Types (|): Allows a variable to be one of several types. Example: type Status = "success" | "error" | "loading";
• Literal Types: Restrict a variable to a specific value or set of values. Example: type Direction = "left" | "right";
• Type Guards and Type Predicates: Functions that check a value’s type at runtime to help TypeScript narrow down the type in that scope.
• Discriminated Unions: A pattern using a common property (often a string literal) to distinguish between different object types in a union.
• Mapped Types: Create new types by transforming properties of an existing type. Example: type ReadOnly<T> = { readonly [P in keyof T]: T[P] };
• Conditional Types:Types that depend on a condition at the type level. Example: type IsString<T> = T extends string ? true : false;
• Indexed Access Types: Allows accessing a property type from another type. Example: type NameType = User["name"];
• Keyof and Lookup Types: keyof gets a union of keys from a type, and lookup types fetch the type of a property. Example: type Keys = keyof Person;
• Utility Types: Built-in helpers like Partial, Pick, Omit, Required, and Readonly that make type transformations easier.

This is a frequent topic in many TypeScript interview questions, as understanding advanced types shows a strong command of TypeScript’s type system and the ability to write robust, type-safe applications.

71. How Do You Define and Use Utility Types in TypeScript?

TypeScript provides several utility types to facilitate common type transformations:

• Partial<T>
• Required<T>
• Readonly<T>
• Record<K, T>
• Pick<T, K>
• Omit<T, K>

interface Todo {
  title: string;
  description: string;
}

// Partial
type PartialTodo = Partial<Todo>;

// Required
type RequiredTodo = Required<Todo>;

// Readonly
type ReadonlyTodo = Readonly<Todo>;

// Record
type StringRecord = Record<string, number>;

let partialTodo: PartialTodo = { title: 'Learn TypeScript' };
let requiredTodo: RequiredTodo = { title: 'Learn TypeScript', description: 'Understand basics' };
let readonlyTodo: ReadonlyTodo = { title: 'Learn TypeScript', description: 'Understand basics' };
// readonlyTodo.title = 'New Title'; // Error: Cannot assign to 'title' because it is a read-only property.
let stringRecord: StringRecord = { key1: 1, key2: 2 };

72. How Can You Use Mapped Types With Conditional Types in TypeScript?

Mapped types can be combined with conditional types to create powerful and flexible type transformations. By iterating over each property in a type and applying a conditional type to decide how each property should be treated, it's possible to construct new types based on specific conditions.

type Nullable<T> = {
  [P in keyof T]: T[P] | null;
};

type ExampleType = {
  a: number;
  b: string;
};

// NullableExampleType properties can be the original type or null
type NullableExampleType = Nullable<ExampleType>;

This example demonstrates how mapped types and conditional types can be used together in TypeScript to transform types dynamically.

Here’s a breakdown:

• The Nullable<T> type iterates over all properties (P in keyof T) of a given type T.
• For each property, it changes the type to either the original type (T[P]) or null.

This is useful when you want to make all properties of a type nullable without altering the original structure. This concept is often tested in TypeScript interview questions, as it shows a developer’s ability to combine multiple advanced type features to achieve dynamic and reusable type transformations.

73. Can You Explain What Template Literal Types Are and When They Are Used in TypeScript?

Template literal types in TypeScript allow you to create string types by combining literal types and expressions, similar to JavaScript's template literals. You can dynamically construct string types using other types within backticks.

Example:

• type Direction = "north" | "south" | "east" | "west";
• type Position = `${Direction}-${number}`;

This means Position can be any string like "north-1", "east-99", etc., where a number follows a direction.

Template literal types are useful when you want to enforce patterns in strings, like forming dynamic API paths or combining constant values with numbers. They help make the code more type-safe and flexible.

It’s considered an advanced feature because it combines TypeScript’s string literal types with template literals, enabling complex type manipulations. This topic frequently appears in TypeScript interview questions, as it demonstrates an understanding of advanced string type manipulation and the ability to create precise, pattern-based type constraints.

74. How Can You Use Mapped Types With Conditional Types in TypeScript?

TypeScript 4.x introduced several improvements over 3.x, refining the language for better usability, performance, and developer experience. Here are the key differences:

• Enhanced Tuple Features:
• Variadic Tuple Types: Allows functions to accept and return dynamic-length tuples.
• Labeled Tuple Elements: Improves clarity by explicitly naming tuple elements.
• Stricter Type System:
• More precise inference and checks: Reducing unintended errors.
• Enhanced control: Over optional properties and function return types.
• Performance & Tooling Improvements:
• Faster compilation: With optimized type-checking.
• Better error messages and diagnostics: For debugging.
• Advanced Editor Integrations:
• Improved auto-completion and IntelliSense: In VS Code.
• Smarter type suggestions: For a better developer experience.
• Safer Handling of unknown and any Types:
• Encourages stricter type safety: Preventing unintended implicit conversions.

75. What is the difference between extends in generics and extends in class inheritance?

In TypeScript, the extends keyword serves different purposes in generics and class inheritance:

• extends in Generics:
• Used to constrain a generic type: Ensuring it must extend (or be compatible with) a specific type.
• Helps enforce type safety: In generic functions, classes, or interfaces.
• extends in Class Inheritance:
• Used to define a parent-child relationship: Between classes.
• Allows a subclass to inherit: Properties and methods from a superclass.

76. How Do You Go About Testing TypeScript Applications on Different Browsers and Devices?

When testing your TypeScript applications across different browsers and devices, it’s essential to ensure your tests cover how your app behaves in various environments. You’ll often create unit tests using libraries like Jest or Mocha to validate your code’s functionality. However, to confirm that your app works consistently across browsers and devices, you need to perform compatibility testing.

Using a cloud-based testing platform can make this process much smoother. These platforms let you run your tests, both manual and automated, across hundreds or even thousands of browser and operating system combinations without the hassle of managing them locally. This approach saves you time and resources while helping you catch browser-specific issues, including edge cases and older browser versions.

One such cloud-based platform is LambdaTest. It enables you to execute tests at scale across 3,000+ browser and OS combinations, supporting popular frameworks like Selenium and Cypress. Integrating this into your workflow helps streamline compatibility testing and ensures your TypeScript app performs reliably everywhere.

77. How Do You Handle Asynchronous Operations in TypeScript?

Handles asynchronous operations using promises and the async/await syntax, similar to JavaScript. TypeScript provides type definitions for promises and async functions.

Using Promises:

function fetchData(): Promise<string> {
  return new Promise((resolve) => {
    setTimeout(() => resolve("Data received"), 1000);
  });
}
fetchData().then((data) => console.log(data)); // Data received (after 1s)

Using async/await for cleaner syntax:

async function getData() {
  const data = await fetchData();
  console.log(data);
}
getData(); // Data received

TypeScript enhances type safety by ensuring proper handling of promise values, making asynchronous operations more predictable and robust. This is a common topic in TypeScript interview questions, as it tests both language syntax and understanding of asynchronous flow control.

78. How Can Async/Await Be Implemented in TypeScript?

In TypeScript, the async and await keywords are used to manage asynchronous operations in a cleaner and more readable way. An async function always returns a Promise, and within that function, the await keyword can be used to pause execution until the Promise resolves.

// a function that returns a Promise 
function delayed Response(): Promise<string> { 
return new Promise(resolve => { 
setTimeout(() => resolve("Hello after 2 seconds"), 2000); 
}); 
} 
async function mainAsync() { 
const response = await delayed Response(); 
console.log(response); // Output: "Hello after 2 seconds" 
} 
mainAsync();

This often appears in TypeScript interview questions because it demonstrates both the syntax and the best practices for managing asynchronous code in a type-safe way.

79. How Can You Create a Custom Decorator in TypeScript?

Decorator is a special type of declaration that can be attached to a class, method, or property to extend or modify its behavior. To create a custom decorator, define a function with the corresponding parameters.

function logClass(constructor: Function) { 
console.log("Class defined: ", constructor.name); 
} 
@logClass 
class Vehicle { 
constructor(private wheels: number) {} 
}

This is a favorite in TypeScript interview questions, especially for advanced candidates, as decorators are a powerful feature used in frameworks like Angular to add metadata or modify class behavior.

Preparing for technical interviews requires a solid understanding of both fundamental and advanced concepts. These carefully selected TypeScript interview questions are designed to help you evaluate your knowledge, identify areas to improve, and gain confidence in applying TypeScript in real-world projects. Regular practice with these TypeScript interview questions will strengthen your readiness for interviews and professional challenges ahead.