What are utility types like Partial, Pick and Omit?

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**Utility types** are built-in generic types that transform existing types, so you derive new types instead of redefining them. They keep types DRY and in sync with their source. ```ts interface User { id: number; name: string; email: string; age: number; } Partial; // all properties optional → for update payloads Required; // all properties required Readonly; // all properties readonly Pick; // { id, name } — keep only these Omit; // everything except id → for create payloads ``` ## Why deriving beats redefining ```ts // Update endpoint: accept any subset of fields function update(id: number, changes: Partial) { ... } update(1, { name: "New" }); // ✅ only the fields you change // Create endpoint: everything except the server-generated id type CreateUser = Omit; ``` If you add a field to `User`, all of these update automatically — no second definition to forget. ## More handy ones ```ts Record; // { [key: string]: number } — dictionaries ReturnType; // the return type of a function Parameters; // tuple of a function's parameter types NonNullable; // string (strips null/undefined) Extract; Exclude; // filter members of a union ``` ## Why it matters Utility types let you express relationships between types ("the create form is the user minus the id") declaratively. They reduce duplication, keep derived types automatically consistent with their source, and are everywhere in real codebases — especially `Partial` for updates, `Omit`/`Pick` for DTOs, and `Record` for maps.