It’s a valid question. A “source map” is a special file that connects a minified/uglified version of an asset (CSS or JavaScript) to the original authored version. Say you’ve got a filed called _header.scss that gets imported into global.scss which is compiled to global.css. That final CSS file is what gets loaded in the browser, so for example, when you inspect an element in DevTools, it might tell you that the
is display: flex; because it says so on line 387 in global.css.
On line 528 of page.css</, we can find out that .meta has position: relative; But because that final CSS file is probably minified (all whitespace removed), DevTools is likely to tell us that we'll find the declaration we're looking for on line 1! Unfortunate, and not helpful for development.
That's where source maps come in. Like I said up top, source maps are special files that connect that final output file the browser is actually using with the authored files that you actually work with and write code in on your file system.
Typically, source maps are a configuration option from the preprocessor. Here's Babel's options . I believe that with Sass, you don't even have to pass a flag for it in the command or anything because it produces source maps by default.
So, these source maps are for developers . They are particularly useful for you and your team because they help tremendously for debugging issues as well as day-to-day work. I'm sure I make use of them just about every day. I'd say in general , they are used for local development. You might even .gitignore them or skip them in a deployment process in order to serve and store fewer assets to production. But there's been some recent chatter about making sure they go to production as well.
David Heinemeier Hansson :
But source maps have long been seen merely as a local development tool. Not something you ship to production, although people have also been doing that, such that live debugging would be easier. That in itself is a great reason to ship source maps. [...]
Additional, Rails 6 just committed to shipping source maps by default in production , also thanks to Webpack. You'll be able to turn that feature off, but I hope you won't. The web is a better place when we allow others to learn from our work.
Check out that issue thread for more interesting conversation about shipping source maps to production. The benefits boil down to these two things:
It might help you track down bugs in production more easily
It helps other people learn from your website more easily
Both are cool. Personally, I'd be opposed to shipping performance-optimized code for learning purposes alone. I wrote about that last year :
I don't want my source to be human-readable, not for protective reasons, but because I care about web performance more. I want my website to arrive at light speed on a tiny spec of magical network packet dust and blossom into a complete website. Or do whatever computer science deems is the absolute fastest way to send website data between computers. I'm much more worried about the state of web performance than I am about web education. But even if I was very worried about web education, I don't think it's the network's job to deliver teachability.
Shipping source maps to production is a nice middle ground. There's no hit on performance (source maps don't get loaded unless you have DevTools open, which is, IMO, irrelevant to a real performance discussion) with the benefit of delivering debugging and learning benefits.
The downsides brought up in recent discussion boil down to:
Sourcemaps require compilation time
It allows people to, I dunno, steal your code or something
I don't care about #2 (sorry), and #1 seems generally negligible for a small or what we think of as the average site, though I'm afraid I can't speak for mega sites.
One thing I should add though is that source maps can even be generated for CSS-in-JS tooling, so for those that literally inject styles into the DOM for you, those source maps are injected as well. I've seen major slowdowns in those situations, so I would say definitely do not ship source maps to production if you can't split them out of your main bundles. Otherwise, I'd vote strongly that you do.
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