immer

Immer is a JavaScript library designed to simplify working with immutable state by allowing developers to write code that mutates a draft state, while Immer produces a new immutable state. Created by Michel Weststrate and first released in 2018, it is widely used in modern JavaScript applications to manage complex state updates with a concise, imperative style.

The core idea is to use proxies to create a draft of the base state. When you pass a recipe function, you can mutate the draft as if it were mutable. Immer records these mutations and returns a new state that preserves immutability by applying only the necessary changes (structure sharing). If the draft is never mutated, the original state is returned. By default, produced states can be auto-frozen to enforce immutability, though this can be configured.

The primary API is the produce function, which takes a base state and a recipe that mutates a draft, returning the next state. For advanced use, Immer also provides createDraft and finishDraft to work with drafts directly, isDraft to check draft status, and current to read values from drafts. An optional patches feature (enablePatches and produceWithPatches) enables generation of JSON patches and inverse patches that describe changes.

Use cases include React state updates and Redux workflows, where Immer can simplify nested updates and reduce the need for manual immutability logic. Limitations include that Immer is not a universal replacement for all immutable data strategies and may have caveats with non-plain objects or certain performance considerations. It integrates well with other libraries and can be toggled to suit various project requirements.