Abstract: Open-source software libraries for audio/music analysis and feature extraction have a significant impact on the development of Audio Signal Processing and Music Information Retrieval (MIR) systems. Despite the abundance of such tools on the native computing platforms, there is a lack of an extensive and easy-to-use reference library for audio feature extraction on the Web. In this paper, we present Essentia.js, an open-source JavaScript (JS) library for audio and music analysis on both web clients and JS-based servers. Along with the Web Audio API, it can be used for efficient and robust real-time audio feature extraction on the web browsers. Essentia.js is modular, lightweight, and easy-to-use, deploy, maintain, and integrate into the existing plethora of JS libraries and Web technologies. It is powered by a WebAssembly back-end of the Essentia C++ library, which facilitates a JS interface to a wide range of low-level and high-level audio features. It also provides a higher-level JS API and add-on MIR utility modules along with extensive documentation, usage examples, and tutorials. We benchmark the proposed library on two popular web browsers, Node.js engine, and Android devices, comparing it to the native performance of Essentia and Meyda JS library.