Source maps link the minified code to the un-minified source code. The minified code includes a special comment which points to the source map.
The browser follows the
sourceMappingURL to download the source map which means
the map needs to be available. The best idea is to store the source map in the
same directory as the minified file which is what most (if not all) JS libs expect.
The source map points to the original source (which also must be available somewhere) and contains the mapping from the minified code to the original. This mapping enables you to debug the minified version in the browser while viewing the original (and hopefully readable) source code.
The way source maps are implemented puts two undesirable restrictions on us.
1. Manage two additional files per library
That means we need three files to be publicly available for each minified JS lib that we want to debug: the minified version, the source map, and the original source.
We manually download all external JS libs into the
assets/vendor/js folder and
commit them directly into the website’s repo.1 This was okay when each JS lib
required only one file to be downloaded; somehow 3 makes a big difference.
2. Can’t change the file names
We prefer to include the version number of the library in the filename,
jquery-2.0.0.min.js so that we can quickly tell which version we’re using.
And it would allow us to use multiple versions of a library if we ever needed to.2
But mostly it makes management of the libraries easier.
Changing file names can break source maps by breaking the pointers stored in the minified file to the source map and in the source map to the original source.
Others feel the pain
These issues are so painful that jQuery stopped enabling source maps by default in the 1.11 and 2.1 releases. jQuery suggests that developers edit the minified file by hand to add the source map comment if they need it. That works well for something like jQuery which you don’t normally update often nor need to debug on your website but that’s not my situation. Editing files by hand makes point #1 even more painful.
I would prefer to see one of jQuery’s suggestions implemented by browsers:3
Make it possible for users to specify a map file manually via a URL in dev tools. This would allow us to ship a file without a sourcemap comment but the user could add the association themselves if they use maps.
That would solve a lot of my pain with source maps because I don’t actually need them that often. I only need them when diagnosing bugs that surface in our error logs. I open the URL where the error occurred on our site, find the conditions that reproduce the error and isolate it the most simple and quickly reproducible situation, then use the debugger to diagnose it. It’s not until I open the debugger that I actually need the source map. Perhaps then I should follow jQuery’s lead and not have them enabled by default but just include them as necessary.