Object identity and deliberate nested updates

The idea

Two variables can refer to the same object, and equality tests that identity rather than comparing all fields. An array or object spread creates a new outer container, but nested objects remain shared. Before updating a record, draw the references: identify the outer object, each nested object, and which aliases can observe a mutation.

For an immutable update, replace the containers along the changed path and retain unaffected references deliberately. This approach preserves the original without claiming that every nested value was deeply copied. It also makes tests precise: verify both the changed result and the unchanged input. Data copied from a boundary still needs validation; copying is an ownership decision, not proof that its fields are valid.

const old = {id: 1, settings: {volume: 3}};
const next = {...old, settings: {...old.settings, volume: 5}};
console.log(old.settings.volume, next.settings.volume);
console.log(old.settings === next.settings);

3 5
false

The inner spread makes a distinct settings object before replacing volume.