Explain Immer for Ergonomic Immutable Updates in React

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Stateintermediate

Immer for Ergonomic Immutable Updates

Learn how Immer lets you write mutation-style code that produces safe, immutable state updates behind the scenes.

Immer is a library that lets you write state update logic as if you were directly mutating a draft object, while it actually produces a brand-new immutable object under the hood using structural sharing. Its core function, produce(baseState, recipe), gives your recipe function a special 'draft' proxy to mutate freely, then returns a new state object reflecting those changes, leaving baseState completely untouched.

Immer is like giving someone a whiteboard overlay to scribble corrections on top of an original printed document — they can mark it up however feels natural, and afterward you get a freshly printed final document reflecting those edits, while the original printed page underneath stays completely unmarked.

Key Concepts

This solves the ergonomic pain of deeply nested immutable updates: updating a deeply nested field the traditional way requires spreading every level of the path ({...state, a: {...state.a, b: {...state.a.b, c: newValue}}}), which is verbose and error-prone to get exactly right. With Immer, the same update is simply draft.a.b.c = newValue.

{...state, a: {...state.a, b: {...state.a.b, c: newValue}}}draft.a.b.c = newValue

Redux Toolkit uses Immer internally for exactly this reason — inside a createSlice reducer, you write state.items.push(x) and Immer ensures the actual Redux store state is updated immutably behind the scenes, combining the mental simplicity of mutation with the correctness guarantees immutability provides for React's change detection and Redux's architecture.

createSlicestate.items.push(x)

Interviewers ask about Immer to check whether a candidate understands it's a change in *authoring ergonomics*, not a change in the underlying immutability contract — the resulting state is still a new object each time, Immer just handles producing it for you via a Proxy-based draft mechanism.