The .map files that are shipped with some javascript libraries like angular are js and CSS files that have been minified. They are called SourceMaps. When a file like angular.js is minified it takes all of the lines of code and turns it to few lines of code. While in production when your app encounters an error the sourcemap will help you to take that few lines of code and allows you to see the original version of the code. If you don't have sourcemap you cannot see the original code. So the source map is to de-reference the minified or uglified code. You can debug an app in production. If you make any changes to the original file the changes will be automatically saved into the minified file.