In Redux, Zustand, immutability means that the state should never be changed directly. Instead of modifying the existing state, you create a new copy of the state with the desired changes.

Similar to Redux, Redux Toolkit and other state management libraries, in Zustand also we need to perform immutable state updates because it relies on predictable state management, and immutability is a core principle that enables this predictability.

Here’s why immutable updates are essential:

1. Zustand/Redux Depends on Reference Comparisons

Redux (especially tools like React-Redux) and Zustand uses shallow reference checks to determine if the state has changed.

• If you mutate/update the existing state object/array, its reference remains the same.

• Redux/Zustand will think nothing has changed, even if the data inside has changed.

• This can lead to components using the selectors not re-rendering when they should.

✅ Immutable update (correct):

state = { users: ['Alice', 'Bob'] };

// Return a new array, not a mutation

return { ...state, users: [...state.users, 'Charlie'] };

❌ Mutating (wrong):

state.users.push('Charlie'); // Mutates the original array
return state; // Same reference → Redux/Zustand thinks nothing changed

In case of zustand, instead of directly mutating state like this:

 add: (user) => {
  const state = get(); // get entire store state
  state.users.push(user); // directly update users property of state
  set(state);
}

// OR

add: (user) => {
  set((state) => {
    state.users.push(user); // directly update users property of state
    return state;
  });
}

write it like this:

set({
  users: [...get().users, user],
});

// OR

addUser: (user) => {
  set((state) => {
    return {
      users: [...state.users, user],
    };
  });
}

2. Time Travel In DevTools

Redux DevTools configured using devtools imported from zustand/middleware can record every state change and let you "time travel" between actions.

• This only works reliably if each state is a unique object.

• If you mutate state, previous "snapshots" get corrupted because they share the same references.

3. Predictable Debugging

Immutable updates make state changes traceable and debuggable:

• Each state is a snapshot.

• You can compare old vs. new state easily.

• No hidden mutations make bugs easier to track.

4. Supports Pure Components & Performance Optimizations

Zustand/Redux uses shallow equality checks to prevent unnecessary re-renders.

• If state is updated immutably, the new state has a different reference → component knows to re-render.

• If state is mutated, reference stays the same → component may skip re-rendering → UI doesn't update.

This makes code more testable, predictable, and maintainable.

✅ How to Update State Immutably

Instead of

state.users.push(newuser); // ❌ mutation

use

return { ...state, users: [...state.users, newuser] }; // ✅ new object/array

Summary:

Immutable updates ensure that Zustand/Redux can detect changes, enable powerful debugging, support performance optimizations, and maintain predictable state behavior.

That’s why you must never mutate state directly, instead always return new objects/arrays when updating.