TypeScript has become an essential tool for modern web development, offering a powerful way to write safer and more maintainable JavaScript. As a superset of JavaScript, TypeScript adds static typing, advanced tooling, and improved developer experience, making it highly favored in enterprise and large-scale applications. For hiring managers and developers alike, preparing for interview questions on TypeScript can help identify the right talent for a role and ensure confidence in handling complex frontend or backend projects.
Basic TypeScript Interview Questions
1. What is TypeScript?
TypeScript is a statically typed superset of JavaScript that compiles to plain JavaScript. It introduces optional typing and enables developers to catch errors early during development. TypeScript helps improve code quality, maintainability, and scalability, especially in larger codebases.
2. How is TypeScript different from JavaScript?
While JavaScript is dynamically typed, TypeScript allows for static typing. This means types are checked during compile time in TypeScript, whereas in JavaScript, type-related errors are typically found only at runtime. TypeScript also includes features like interfaces, enums, access modifiers, and advanced IDE support.
3. What are the benefits of using TypeScript?
- Early detection of bugs through type checking
- Better IDE support with code completion and navigation
- Improved readability and self-documentation
- Supports modern JavaScript features with backward compatibility
- Easier refactoring in large codebases
4. How do you install TypeScript?
TypeScript can be installed globally using npm:
npm install -g typescriptOnce installed, thetsccommand can be used to compile TypeScript files into JavaScript.
Intermediate TypeScript Interview Questions
5. What are interfaces in TypeScript?
Interfaces define the structure of an object by specifying the expected properties and their types. They help enforce a consistent shape and can be used for function parameters, object definitions, and class contracts.
interface User { name: string; age: number; }6. What is the difference between type aliases and interfaces?
Both can be used to define custom types. However, interfaces are best for describing the shape of objects and support declaration merging. Type aliases are more flexible and can be used for primitives, unions, and intersections.
type ID = string | number; interface User { name: string; }7. How does TypeScript support classes and inheritance?
TypeScript adds features like access modifiers (public, private, protected), constructors, and inheritance similar to object-oriented languages. You can create subclasses using theextendskeyword.
class Animal { constructor(public name: string) {} } class Dog extends Animal { bark() { console.log(`${this.name} barks`); } }8. What are enums in TypeScript?
Enums allow a developer to define a set of named constants. TypeScript supports numeric and string enums.
enum Direction { Up, Down, Left, Right }9. What are generics in TypeScript?
Generics provide a way to create reusable components that work with a variety of data types without sacrificing type safety.
function identity<T>(arg: T): T { return arg; }Advanced TypeScript Interview Questions
10. What is type inference?
TypeScript automatically infers the type of a variable based on its assigned value. This reduces the need for explicit type annotations while still providing type safety.
let message = 'Hello'; // inferred as string11. Explain union and intersection types in TypeScript.
- Union Typesallow a variable to be one of several types:
type Result = string | number; - Intersection Typescombine multiple types into one:
type Employee = Person & Worker;
12. What are mapped types?
Mapped types create new types by transforming properties of an existing type.
type ReadOnly<T> = { readonly [P in keyof T]: T[P]; };13. What is the difference between any, unknown, and never?
- any: disables type checking completely
- unknown: safer alternative to any; must be type-checked before use
- never: represents values that never occur, such as a function that throws
14. How do you use utility types in TypeScript?
TypeScript provides several built-in utility types such as:
Partial<T>: makes all properties optionalPick<T, K>: creates a type with a subset of propertiesOmit<T, K>: removes specified properties
Scenario-Based Interview Questions
15. How would you migrate a JavaScript project to TypeScript?
Steps include:
- Install TypeScript and initialize
tsconfig.json - Rename
.jsfiles to.ts - Fix type errors gradually by adding annotations
- Leverage
anyas a temporary solution - Introduce types and interfaces for commonly used objects
16. How do you handle third-party JavaScript libraries in TypeScript?
Use DefinitelyTyped type definitions or create custom.d.tsdeclaration files to describe the types used in the library.
17. What are declaration files?
Declaration files (.d.ts) provide type information about JavaScript code, allowing TypeScript to understand types from libraries or non-TypeScript modules.
18. How do you enforce strict typing in a TypeScript project?
Enable the'strict': trueflag intsconfig.json. This activates all strict type-checking options likenoImplicitAny,strictNullChecks, andstrictFunctionTypes.
Best Practices Questions
19. How do you structure a large-scale TypeScript project?
Key practices include:
- Modular file structure with proper folders
- Using interfaces and types to enforce contracts
- Applying generics to reusable components
- Documenting code with JSDoc or inline comments
- Using a linter like ESLint with TypeScript rules
20. How do you debug TypeScript code?
Use source maps by enabling'sourceMap': trueintsconfig.json. This allows debugging original TypeScript files in browsers or IDEs like VS Code. Additionally, modern tools like ts-node can help run and debug TypeScript directly without compilation.
Preparing for a TypeScript Interview
Mastering TypeScript interview questions requires a solid understanding of both the language features and how they solve real-world problems in development. From basic type safety to advanced type manipulation and project structuring, being well-versed in TypeScript not only improves your chances during interviews but also enhances your development workflow. Employers look for candidates who can write clean, scalable, and bug-resistant code using TypeScript, making preparation on these questions essential for any frontend or full-stack developer role.