SonarJS is a static code analyzer for JavaScript, TypeScript, and CSS that helps developers identify bugs, vulnerabilities, and code smells in their code. It offers around 280 rules for JavaScript and TypeScript, and 30 rules for CSS, along with features like complexity metrics, test coverage import, and support for modern ECMAScript versions and frameworks like React JSX and Vue. This tool is ideal for developers and teams using SonarQube to maintain code quality and adhere to best practices in their projects.