 Today, we are talking about maps, but not this map, it's the source maps. Source maps are files with names ending in .map. They can be generated by most built tools, with some tools including them by default, while others may need additional configuration to produce them. Let's take a quick look at the JavaScript source map. Here is the interesting bit. The sources field hold a list of original sources used by the mappings entry. In our case, there's only one file, but it can be more. The mappings fields is where the real magic occurs. It tells us how to map lines and locations in the compiled file to the original files.