Debug compiled CSS and JS in Firefox using sourcemaps

If you haven’t used sourcemaps, now is the time!

With the advent of compiled CSS like LESS or SASS, or minified/uglified JS, it makes debugging awkward as you cant see the original lines of code that you wrote. Cue CTRL_F’ing to find that line to tweak! But not any more!

When you generate your LESS/SASS, if you tell it to compile a sourcemap too, it will create one! This allows your browser to report which line of the less or scss file the rule in question came from.

For the Less compiler command, you have several options:

--source-map[=FILENAME]  Outputs a v3 sourcemap to the filename (or output filename.map)
--source-map-rootpath=X  adds this path onto the sourcemap filename and less file paths
--source-map-basepath=X  Sets sourcemap base path, defaults to current working directory.
--source-map-less-inline puts the less files into the map instead of referencing them
--source-map-map-inline  puts the map (and any less files) into the output css file
--source-map-url=URL     the complete url and filename put in the less file

For Sass, you just add the option at the end of the command:

sass sass/screen.scss:stylesheets/screen.css --sourcemap

Now in Firefox, open your page, right click, and click Inspect Element (not the Firebug one, the default Firefox Inspector!)

In the CSS rules pane, right click, and make sure show original sources is checked.

Screen Shot 2015-05-18 at 16.57.27 (2)

To the right of the actual rule, you click the link which would usually take you to the CSS file for viewing, but now, it takes you to your Less/Sass file! You can now tweak rules and save them directly from the inspector!

Screen Shot 2015-05-18 at 17.01.11

In my new workplace, our SASS files are compiled by Gulp automatically. (In my last work I had lessc configured in PHPStorm). If your own files are automatically built, then you should also checkout Firefox addons for live reloading! The great thing about that is that the CSS files update without refreshing! Great if you have session info or other info that might need reinstantiating, you dont have to keep keying stuff in! Have fun!

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