use-utility-types-for-dry-types

Summary

Do not manually redefine interfaces that are subsets or variations of existing types. Use TypeScript's built-in Utility Types (Pick, Omit, Partial, Required, ReturnType) to derive new types from a "Single Source of Truth".

Rationale

• Maintenance: When the source type changes (e.g., adding a field to User), derived types update automatically.
• Consistency: Prevents subtle bugs where a DTO and the Domain Model diverge over time.
• Intent: Seeing Pick<User, 'id' | 'email'> clearly communicates "This is just a user ID and email" better than a generic UserDTO.

Guidance

1. Derive Downwards: Define the "full" entity (e.g., database model) first, then derive input/output types from it.
2. Avoid Duplication: If you catch yourself copy-pasting properties, stop and use a Utility Type.
3. Composition: You can combine them: Partial<Pick<User, 'name'>>.

Examples