Immutablejs

Immutable.js is a JavaScript library that provides persistent immutable data structures. It offers a collection of immutable types, including Map, List, Set, OrderedMap, Record, Seq and Range, which mirror their mutable equivalents but cannot be altered once created. Operations on Immutable collections return new collections rather than mutating existing ones. Structural sharing ensures that unchanged parts of the data are reused, making updates relatively efficient for large structures.

Immutable.js emphasizes predictable state handling and functional programming patterns. Key features include path-based access and updates (get, set, getIn, setIn, update, updateIn), convenient constructors (Map({ ... }), List([...]), Record({ ... })), and equality semantics based on content. Two immutable structures are considered equal when they contain the same data, and the library provides an is() function for value-based equality.

Values can be converted to plain JavaScript via toJS(), and Immutable objects can be serialized or interpolated with JSON after explicit conversion. The API is designed to be familiar to JavaScript developers, with methods that resemble native counterparts while returning new instances.

Common use cases include managing application state in React or Redux apps, where immutability helps with change detection and makes debugging easier. Limitations include a learning curve and potential performance trade-offs, as immutability introduces indirection and may require conversions for interoperability with non-Immutable libraries.

Immutable.js is an open-source project maintained on npm and widely adopted in JavaScript ecosystems.