immerjs/immer

Immer is a JavaScript library that makes it easier to work with immutable data, data that should not be directly changed once created. Immutability is a common pattern in JavaScript applications, particularly those using state management tools like Redux, but it typically requires verbose code where you manually copy every level of a nested object before making a change. Immer solves this problem by letting you write code that looks like you are directly modifying an object, setting a property, pushing to an array, while behind the scenes it tracks those changes and produces a brand-new, updated copy without touching the original. You get the readability of direct mutation with the safety of immutability. The core concept is a function where you receive a "draft" copy of your state, make changes to it using normal assignment syntax, and Immer handles creating the final immutable result. If none of your changes actually differ from the original, Immer returns the original object unchanged, which helps with performance in frameworks that use reference equality checks to decide what to re-render. It is especially useful when writing reducers (functions that update application state in response to user actions) or any code that manages deeply nested objects. The library works with plain JavaScript and TypeScript.