Immutability

Immutability means you never modify existing state. Each update produces a new state object.

Why it matters

• Reliable change detection via reference checks
• Predictable updates and debug traces
• Enables memoization and time-travel tooling

In Redux and RTK

• Reducers must return new objects
• RTK uses Immer to allow mutation-like syntax while keeping immutability

// correct
return { ...state, count: state.count + 1 }