$ show immerjs/immer
Immer is a zero-dependency TypeScript library that enables immutable state updates through mutable-style code. Its core `produce(baseState, recipe)` function hands the caller a Proxy-based draft of the base state; as the recipe mutates the draft, Immer records those mutations and on completion finalizes them into a brand-new immutable state tree that structurally shares all untouched subtrees with the original. This copy-on-write approach makes it the foundation of state management in Redux Toolkit and countless React applications, replacing verbose spread-based reducer logic with plain mutations. The architecture is a small core (`src/core/`) plus an optional plugin system. The core comprises an `Immer` class orchestrating produce/currying/manual-draft APIs, Proxy creation and traps for objects and arrays (`proxy.ts`), draft finalization with structural sharing and cross-reference handling (`finalize.ts`), and per-call scope management (`scope.ts`). Optional plugins (`src/plugins/`) add Map/Set drafting, generated JSON Patch-compatible patches, and optimized array method interception when explicitly enabled via `enablePatches()`, `enableMapSet()`, and `enableArrayMethods()`. The public entry point `src/immer.ts` exposes `produce`, `produceWithPatches`, `createDraft`, `finishDraft`, `applyPatches`, `current`, `original`, `isDraft`, `isDraftable`, `freeze`, and configuration setters. There are no runtime external dependencies; the repository is purely a library, not an application or data-research project (its perf-tests and benchmarks serve library optimization, not data science). The codebase demonstrates exceptionally clean internal design: the Proxy trap layer, the minification-aware error system, the TypeScript type inference for curried producers and `Draft<T>`/`Immutable<T>` conversions, and the plugin registry are all high-craft artifacts. Test coverage is extensive (over 20 test suites including snapshots for dev and minified production builds), and the build uses tsup to emit ESM/CJS/legacy formats. The repository is actively maintained and current with its ecosystem. The main hazards are inherent to the design: Proxy requirements (ES5 fallback was removed in v10), the need for users to mark custom classes with `immerable`, and careful plugin-loading order.
The LLM identified 4 files worth highlighting for reuse:
Implements optional patch generation and application for Immer drafts. It produces JSON Patch-like add/replace/remove operations by diffing the finalized draft against the base state, handles nested patch paths, and integrates with the finalizer's cross-reference tracking. The algorithm is self-contained, standard-conformant, and could be extracted for any diff/patch use case beyond Immer's produce flow.
Adds Proxy-based drafting for Map and Set collections, intercepting mutations like set/delete/add/clear and recording them for finalization. Demonstrates advanced Proxy handler design for non-plain-object targets, including iteration and size tracking against the draft state. This pattern is reusable for any library needing transparent immutable wrappers around mutable collections.
The heart of Immer: creates lazily-initialized Proxy drafts for objects and arrays, with separate trap sets (`objectTraps`, `arrayTraps`) that log mutations onto the draft's internal state and hide draft-only bookkeeping properties. This is a masterclass in Proxy metaprogramming — handling get/set/has/ownKeys/deleteProperty/defineProperty correctly for both plain objects and arrays — and is the canonical reference implementation for copy-on-write drafts.
A tiny, elegant plugin registry that lets Immer ship optional features (Map/Set, patches, array methods) as opt-in plugins. It manages plugin storage per Immer instance, version checks, and enables lazy loading with clear error messages when an unloaded plugin is used. This pattern is a clean template for any library wanting zero-cost feature gating without bundler tree-shaking complexity.