Answer

Utility types are built-in TypeScript tools for transforming, modifying, or deriving types based on existing ones. The most commonly encountered:

• Partial<T> — makes all properties of type T optional
• Required<T> — makes all properties mandatory
• Pick<T, K> — selects properties K from type T only
• Omit<T, K> — excludes properties K from type T

Example usage:

type User = { id: number; name: string; age?: number };

type UserPreview = Pick<User, 'id' | 'name'>;
type UserPatch = Partial<User>;
type FullUser = Required<User>;
type UserWithoutAge = Omit<User, 'age'>;

This allows for creating more flexible APIs and DTOs.

Trick question

If a property of the type is specified as optional (age?: number), how will Required<T> behave? Will it remain optional?

Answer: No, Required<T> transforms all properties of type T, including optional ones, into mandatory (required).

type User = { age?: number };
type UserRequired = Required<User>;
// UserRequired: { age: number }

Examples of real mistakes due to lack of understanding of the nuances

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Story

Used Partial<T> for updating objects, thinking it was automatically safe for the database. Forgot to check fields for null/undefined and faced validation errors when updating entities.

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Story

Used Omit<T, K> to exclude technical properties from the request type, not considering that during model refactoring new technical fields were not added to the exclusion list. As a result, private fields "leaked" through the API.

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Story

In the Shared library, created type Pick<T, K>, making a mistake in the list of keys: K was partially missing from the original type. TypeScript missed the error when using literal strings, but when the original type changed, there was a mismatch, and part of the types "broke" across all services.