readonly — is the object immutable at runtime?

readonly blocks assigning to a property through that typed reference. It doesn't freeze the object at runtime and isn't deep by default.

interface Settings {
  readonly id: string;
  nested: { theme: string };
}
const settings: Settings = { id: 's1', nested: { theme: 'light' } };
// settings.id = 's2'; // TypeScript error
settings.nested.theme = 'dark'; // allowed

To be precise: Readonly<T> is shallow too. Object.freeze works at runtime, but it also only freezes the object's own properties.

Pitfall: readonly is a usage contract in the types, not full protection against mutation through another alias.