A JavaScript minifier reduces the size of JavaScript files by removing unnecessary characters such as extra spaces, line breaks, and comments while preserving the code's functionality. Smaller JavaScript files generally require less data to be transferred when a webpage loads. This can be useful when preparing code for a production website. Developers should keep the original, readable source code separately because minified code can be harder to edit or troubleshoot. After minification, it is also important to test the website to make sure scripts continue to work correctly.