Partial, Pick, and Omit

Types made from other types

TypeScript ships utility types, generic types that transform existing ones. They exist because real projects constantly need variations of one shape — the same user with fields optional for an update form, or with private fields removed for display — and hand-writing each variation means shapes that drift out of sync. The ones you will use weekly:

The classic Partial use case is an update function. Callers change one setting, not all three:

interface Settings {
  theme: string;
  fontSize: number;
  autosave: boolean;
}

function applyChanges(base: Settings, changes: Partial<Settings>): Settings {
  return { ...base, ...changes };
}

The spread syntax from Advanced JavaScript merges the objects, and the types guarantee changes only ever contains valid Settings properties.

Readonly<T> applies lesson 3-2's readonly to every property at once, for values that must never be mutated after creation.

Pick and Omit: sub-shapes without duplication

The server behind a web API should never send passwordHash to the browser. Instead of writing a second interface by hand and keeping it in sync forever, derive it:

interface User {
  id: number;
  name: string;
  email: string;
  passwordHash: string;
}

type PublicUser = Omit<User, "passwordHash">;
type UserPreview = Pick<User, "id" | "name">;

PublicUser has everything except the hash. UserPreview has only id and name, note the union of literal property names from Unit 4. When User gains a field, the derived types update themselves.