TypeScript Interview Questions

Q: What are utility types?

Built-in type transformers: Partial , Required , Pick , Omit , Record , Readonly , ReturnType . Partial (all optional), Required (all required), Pick (select properties), Omit (exclude), Record (key-value map), Readonly (immutable), ReturnType, Parameters, Extract, Exclude, NonNullable. Build custom utilities with mapped and conditional types.

Question 1

What is TypeScript?

Accepted Answer

TypeScript is a typed superset of JavaScript adding static types for better tooling, error detection, and code maintainability. Adds: type annotations, interfaces, enums, generics, union/intersection types, type guards. Benefits: compile-time errors, IDE autocomplete, self-documenting code. Compiles to any JS version. Used by Angular, React, Vue, and most modern frameworks.

Question 2

What is the difference between interface and type?

Accepted Answer

Interfaces define object shapes and support declaration merging; types support unions, intersections, and mapped types. interface: declaration merging, extends for inheritance. type: union (A|B), intersection (A&B), mapped types, conditional types. Both describe objects. Best practice: interface for object shapes, type for unions and complex types.

Question 3

What are generics?

Accepted Answer

Generics let you write reusable code working with multiple types while maintaining type safety, using parameters. function identity(arg: T): T. Constraints: . Common: Array, Promise, Map. Utility types are generic: Partial, Pick, Omit, Record.

Question 4

What is the difference between any, unknown, and never?

Accepted Answer

any disables type checking; unknown requires narrowing before use; never represents impossible values. any: opt out of type system. unknown: top type, must narrow before use. never: bottom type, for functions that never return or exhaustive checks. Use unknown for API boundaries; any only for migration.

Question 5

What are utility types?

Accepted Answer

Built-in type transformers: Partial, Required, Pick, Omit, Record, Readonly, ReturnType. Partial (all optional), Required (all required), Pick (select properties), Omit (exclude), Record (key-value map), Readonly (immutable), ReturnType, Parameters, Extract, Exclude, NonNullable. Build custom utilities with mapped and conditional types.

Question 6

What are type guards?

Accepted Answer

Type guards narrow types at runtime: typeof, instanceof, in operator, or custom predicate functions with 'is' keyword. typeof: typeof x === 'string'. instanceof: x instanceof Date. in: 'name' in obj. Custom: function isUser(obj): obj is User. Discriminated unions: shape.kind === 'circle'. Enable safe handling of union types.

Question 7

What are mapped types?

Accepted Answer

Mapped types transform properties of existing types: { [K in keyof T]: NewType } — used to build utility types. type Readonly<T> = { readonly [K in keyof T]: T[K] }. Modifiers: +/- for readonly and ?. Key remapping with 'as'. Template literal types with mapped types for derived types.

Question 8

What are conditional types?

Accepted Answer

Conditional types select types based on conditions: T extends U ? X : Y, with infer for extracting types. type IsString = T extends string ? true : false. infer: type ElementType = T extends Array ? U : T. Distributive over unions. Built-in: Exclude, Extract, ReturnType use conditional types.

Question 9

What are template literal types?

Accepted Answer

Template literal types create string types from combinations: type Route = `/api/${string}` for precise string validation. Combine with unions: type Color = 'red'|'blue'; type BgColor = `bg-${Color}`. Built-in: Uppercase, Lowercase, Capitalize. Used with mapped types for property renaming. Enables compile-time string validation.

Question 10

How do you migrate a JavaScript project to TypeScript?

Accepted Answer

Add tsconfig with allowJs and strict:false, rename files incrementally starting with leaf modules, gradually enable strict checks. Steps: (1) tsconfig.json with allowJs:true, strict:false. (2) Rename .js to .ts one at a time. (3) Start with leaf modules. (4) Add types to functions. (5) Use any temporarily. (6) Create .d.ts for third-party JS. (7) Enable strict options gradually. (8) Target: zero 'any'.

Free TypeScript Course - Learn Types, Interfaces & Advanced TypeScript

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Course Modules Overview

Introduction to TypeScript

Type Annotations & Inference

Interfaces & Type Aliases

Functions & Generics

Classes and OOP

Advanced Types & Utility Types

Modules & Namespaces

TypeScript with React

Error Handling & Debugging

TypeScript Project Fullstack App

Advanced TypeScript Patterns

TypeScript Career & Ecosystem

Course Curriculum

Introduction to TypeScript

Type Annotations & Inference

Interfaces & Type Aliases

Functions & Generics

Classes and OOP

Advanced Types & Utility Types

Modules & Namespaces

TypeScript with React

Error Handling & Debugging

TypeScript Project Fullstack App

Advanced TypeScript Patterns

TypeScript Career & Ecosystem

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Beginner

Intermediate

Advanced

Tools & Technologies

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TSC

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