400 Typescript Interview Questions with Answers 2026

Master TypeScript with expert-level practice tests, detailed explanations, and advanced coding patterns.

TypeScript Interview Practice Questions and Answers is a comprehensive, high-impact resource designed to help developers, students, and engineers bridge the gap between basic syntax and production-grade mastery. Whether you are preparing for a rigorous technical interview or a professional certification, this course provides a deep dive into the TypeScript ecosystem, covering everything from core fundamentals like type narrowing and interfaces to complex type-level programming including mapped and conditional types. Each question is crafted to mirror real-world scenarios, ensuring you understand not just the “how” but the “why” behind every line of code. By engaging with these structured domains—ranging from Object-Oriented patterns and SOLID principles to tooling with Vite and runtime validation—you will build the confidence to write safer, more maintainable code and ace any technical screening.

Exam Domains & Sample Topics

• TypeScript Fundamentals: Type inference, narrowing, unions, and utility types.

• Advanced Type Systems: Conditional types, template literals, and mapped types.

• Design Patterns: OOP, functional programming, and dependency injection.

• Tooling & Ecosystem: tsconfig optimization, Monorepos, and React/Node integration.

• Production Practices: Security, Zod validation, and performance profiling.

Sample Practice Questions

Q1: Which TypeScript feature allows you to create a new type by transforming each property of an existing type? A) Type Aliases B) Interface Extension C) Mapped Types D) Intersection Types E) Enum Members F) Ambient Declarations

Correct Answer: C

Overall Explanation: Mapped types allow you to take an existing type and transform its properties into a new type using a syntax similar to the map method in JavaScript.

• A is incorrect: Type aliases simply give a name to a type but do not inherently transform properties.

• B is incorrect: Interface extension adds new properties but doesn’t iterate over existing ones to transform them.

• C is correct: This is the specific purpose of mapped types (e.g., { [P in K]: T }).

• D is incorrect: Intersection types combine multiple types but do not transform individual properties of a base type.

• E is incorrect: Enums define a set of named constants and cannot be used to transform existing types.

• F is incorrect: Ambient declarations describe existing JavaScript code to the compiler and do not perform type transformations.

Q2: In “Strict Mode,” what is the result of attempting to assign null to a variable of type string? A) It is silently ignored. B) The variable becomes undefined. C) A compile-time error occurs.

• D) The variable’s type is automatically changed to any. E) It results in a runtime crash only. F) TypeScript converts null to an empty string “”.

Correct Answer: C

Overall Explanation: When strictNullChecks is enabled (a core part of Strict Mode), null and undefined are not assignable to other types.

• A is incorrect: TypeScript’s compiler will catch this assignment to prevent “billion-dollar mistake” errors.

• B is incorrect: null and undefined are distinct values; one does not become the other.

• C is correct: Strict mode forces you to explicitly handle nullability, triggering a compile-time error for unsafe assignments.

• D is incorrect: Types are static; assigning a value does not change the declared type to any.

• E is incorrect: TypeScript is a static checker; the goal is to catch these errors before the code ever runs.

• F is incorrect: TypeScript never automatically coerces values between types at runtime.

Q3: Which utility type is used to construct a type consisting of all properties of Type set to optional? A) Required<Type> B) Readonly<Type> C) Pick<Type, Keys> D) Partial<Type> E) Omit<Type, Keys> F) Record<Keys, Type>

Correct Answer: D

Overall Explanation: Utility types facilitate common type transformations; Partial is the standard tool for making all properties in an object type optional.

• A is incorrect: Required does the opposite, making all optional properties mandatory.

• B is incorrect: Readonly prevents properties from being reassigned but does not make them optional.

• C is incorrect: Pick selects a specific subset of properties but maintains their original optional/required status.

• D is correct: Partial<T> wraps all properties of T in a ? modifier.

• E is incorrect: Omit removes specific keys entirely rather than making them optional.

• F is incorrect: Record is used to map keys to a specific type, not to modify the optionality of an existing type.

• Welcome to the best practice exams to help you prepare for your TypeScript Interview Practice Questions and Answers.

• You can retake the exams as many times as you want

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I hope that by now you’re convinced! And there are a lot more questions inside the course. Enroll today and take the final step toward getting certified!

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