Object shapes and readonly views

The idea

An object type describes required properties and their value types. Optional properties may be absent, so reading one requires handling undefined under strict checking. Structural compatibility means a value can satisfy a contract by having the needed shape rather than inheriting from a named class. Prefer small contracts that expose only the data an operation actually requires.

Readonly restricts assignments through a particular typed view. It does not freeze the runtime object, and readonly on an outer property does not recursively protect the referenced value. Similarly, readonly arrays prevent array writes through that view but do not necessarily protect fields inside their elements. Treat these tools as useful API signals and combine them with disciplined copying when an operation promises to preserve its input.

type Ticket = { readonly id: number; title: string; owner?: string };
function label(ticket: Ticket): string {
  return ticket.owner ?? "Unassigned";
}
console.log(label({id: 4, title: "Search"}));

Unassigned

The optional owner may be undefined. Nullish coalescing supplies a label, while readonly prevents assigning a new id through this typed parameter.