A JS Minifier is a tool used to reduce the size of JavaScript files by removing unnecessary characters such as white spaces, comments, and new line characters. This process is also called "minification" or "compression".

The primary goal of JS Minifiers is to make JavaScript code smaller and faster to download, which can significantly improve the page loading time of a website or web application. Minified JavaScript files are commonly used in production environments to optimize web performance.

JS Minifier tools typically work by taking a JavaScript source file as input and then removing all unnecessary characters, resulting in a smaller output file. Some minifiers also have additional features like renaming variables and function names to reduce the size of the code further.

There are several popular JS Minifier tools available in the market, including UglifyJS, Closure Compiler, and YUI Compressor. These tools can be used either through a command-line interface or integrated into a build system such as Grunt, Gulp, or Webpack.