How to use JavaScript source maps (.map files)?
Recently I have seen files with .js.map extension shipped with some JavaScript libraries (like Angular), and that just raised few questions in my head:
- What is it for? Why do the guys at Angular care to deliver a
.js.mapfile? - How can I (as a JavaScript developer) use the
angular.min.js.mapfile? - Should I care about creating
.js.mapfiles for my JavaScript applications? - How does it get created? I took a look at
angular.min.js.mapand it was filled with strange-formatted strings so I assume it's not created manually.
Answer
The .map files are for js and css (and now ts too) files that have been minified. They are called SourceMaps. When you minify a file, like the angular.js file, it takes thousands of lines of pretty code and turns it into only a few lines of ugly code. Hopefully, when you are shipping your code to production, you are using the minified code instead of the full, unminified version. When your app is in production, and has an error, the sourcemap will help take your ugly file, and will allow you to see the original version of the code. If you didn't have the sourcemap, then any error would seem cryptic at best.
Same for CSS files. Once you take a SASS or LESS file and compile it to CSS, it looks nothing like its original form. If you enable sourcemaps, then you can see the original state of the file, instead of the modified state.
So, to answer you questions in order:
- What is it for? To de-reference uglified code
- How can a developer use it? You use it for debugging a production app. In development mode you can use the full version of Angular. In production, you would use the minified version.
- Should I care about creating a js.map file? If you care about being able to debug production code easier, then yes, you should do it.
- How does it get created? It is created at build time. There are build tools that can build your .map file for you as it does other files. https://github.com/gruntjs/grunt-contrib-uglify/issues/71
I hope this makes sense.