TypeScript Practice Questions

Practice real questions in TypeScript. Check your answer, understand the reasoning, and build confidence before taking a formal assessment.

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TypeScript Easy

Web Development → TypeScript

What is the result of the following TypeScript code snippet regarding type inference? let value = “Hello”; value = 42;

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TypeScript Hard

Web Development → TypeScript

What does TypeScript’s ‘resolution-mode’ import attribute enable in moduleResolution: bundler mode?

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TypeScript Hard

Web Development → TypeScript

What does the following type produce and how does it work? type Zip = T extends [infer TH, …infer TT] ? U extends [infer UH, …infer UT] ? [[TH, UH], …Zip] : [] : [];

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TypeScript Hard

Web Development → TypeScript

What does the following ‘using’ and ‘Symbol.dispose’ pattern guarantee? class DbConnection { [Symbol.dispose]() { this.close(); } close() { /* cleanup */ } } { using conn = new DbConnection(); // use conn }

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TypeScript Hard

Web Development → TypeScript

What does the following TypeScript 4.1+ template literal type produce? type PropEventSource = { on(event: `${K}Changed`, callback: (value: T[K]) => void): void; };

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TypeScript Hard

Web Development → TypeScript

What does ‘bivariant’ method checking mean and when does TypeScript use it?

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TypeScript Hard

Web Development → TypeScript

What does the following type guard pattern enable that ‘as’ casting does not? function isUser(value: unknown): value is User { return typeof value === ‘object’ && value !== null && ‘id’ in value; }

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TypeScript Hard

Web Development → TypeScript

What does the following declaration merge between a namespace and a class enable? class Animal { name: string = ”; } namespace Animal { export function create(name: string): Animal { const a = new Animal(); a.name = name; return a; } }

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TypeScript Hard

Web Development → TypeScript

What issue does the following TypeScript pattern address? type StrictOmit = Omit;

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TypeScript Hard

Web Development → TypeScript

What does the following generic function demonstrate about TypeScript’s ‘infer in conditional return types’? type Unbox = T extends { value: infer V } ? V : never; type Result = Unbox;

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TypeScript Hard

Web Development → TypeScript

What does the following type-level string manipulation compute? type CamelToSnake = S extends `${infer Head}${infer Tail}` ? Head extends Uppercase ? `_${Lowercase}${CamelToSnake}` : `${Head}${CamelToSnake}` : S;

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TypeScript Hard

Web Development → TypeScript

What does TypeScript’s ‘exactOptionalPropertyTypes’ flag change about the following type? interface Config { timeout?: number; }

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TypeScript Hard

Web Development → TypeScript

What does the following type implement and what is it useful for? type Opaque = T & { readonly __brand: Brand }; type UserId = Opaque; type ProductId = Opaque;

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TypeScript Hard

Web Development → TypeScript

What does TypeScript’s ‘using await’ (async explicit resource management) enable differently from regular ‘using’?

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TypeScript Hard

Web Development → TypeScript

What does the following ‘infer’ usage extract? type PromiseValue = T extends Promise ? PromiseValue : T;

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TypeScript Hard

Web Development → TypeScript

What does the following function overload pattern enable? function process(input: string): string; function process(input: number): number; function process(input: string | number): string | number { return input; }

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TypeScript Hard

Web Development → TypeScript

What does the following type utility compute? type DeepPartial = T extends object ? { [K in keyof T]?: DeepPartial } : T;

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TypeScript Hard

Web Development → TypeScript

What is the difference between ‘interface’ declaration merging and type alias intersection when both add the same property with incompatible types?

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