JavaScript Minifier is an online tool used to compress and optimize JavaScript code. It works by removing unnecessary characters such as spaces, line breaks, and comments, while preserving the functionality of the code. The compressed code produced by the tool is typically smaller in size than the original code, which can result in faster loading times for websites and applications that use JavaScript. In addition to minimizing the code, the tool can also obfuscate it, making it more difficult to read and reverse-engineer. This can be useful for protecting sensitive code and intellectual property.