Log4js is a popular JavaScript logging library used to generate log messages in web applications. It provides an easy way to track and monitor the behavior of an application by recording various events and messages. Log4js offers different logging levels such as debug, info, warn, error, and fatal, allowing developers to categorize and filter log messages based on their severity. With Log4js, developers can configure logging properties, such as log format, output destination (console, file, database), and log rotation. This library supports different appenders and layouts, giving developers flexibility in managing and formatting log data. Log4js is widely used in both frontend and backend development to improve the debugging and troubleshooting process.