Immer does a correct nested immutable update, same as if you wrote the corresponding nested spread operations by hand. It _only_ updates the nesting paths that

Immer does a correct nested immutable update, same as if you wrote the corresponding nested spread operations by hand. It _only_ updates the nesting paths that led to the value that got modified. So yes, if you have a nested object with a bunch of different fields, and you update `state.a.b.c = 123`, it makes copies of `b`, `a`, and `state`, but preserves all other nested references that are anywhere inside of `state`. It's the same as if you wrote: return { ...state, a: { ...state.a, b: { ...state.a.b, c: 123 } } } but obviously much shorter and easier to read / maintain :) It also conveniently eliminates the chance of a _real_ accidental mutation (which in the case of Redux was always the #1 cause of bugs in Redux apps).