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 TopicsTypeScript 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 QuestionsQ1: 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 DeclarationsCorrect Answer: COverall 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: COverall 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: DOverall 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 wantThis is a huge original question bankYou get support from instructors if you have questionsEach question has a detailed explanationMobile-compatible with the Udemy app30-day money-back guarantee if you're not satisfiedI 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!