uglify-js is a JavaScript parser, mangler, and compressor/beautifier toolkit. It's invaluable for optimizing JavaScript code for production, reducing file size by shortening variable names and removing whitespace. It supports ES6+ syntax and offers extensive customization options through its API, allowing fine-grained control over the minification process. Popular for optimizing web performance, uglify-js is a widely used choice for JavaScript developers aiming for smaller, faster web applications.