Announcing Dart Sass

Over the past few months, I've been quietly working on a new project. Today I'm ready to announce Dart Sass to the world. It's a totally new implementation of Sass, designed to be fast, easy to install, and easy to hack on. It's not yet complete—I'm steadily working my way through sass-spec—so today I'm just releasing version 1.0.0-alpha.1. But it's solid enough for you to download, play with, and start filing issues.

You can download a standalone archive from the release page—just extract it, add the folder to your path, and run dart-sass. Dart also compiles to JavaScript, so if you have npm installed you can install the JS version by running npm install -g dart-sass. And, if you happen to be a Dart user yourself, you can install it using pub global install sass.

Why Rewrite Sass?

Over the past few years, there have been two primary implementations of Sass. Ruby Sass was the original, written mostly by me with substantial help from Chris. It's high-level and easy to hack on, so it's where we iterate on new features and where they first get released. Then there's LibSass, the C++ implementation, originally created by Aaron and Hampton and now maintained by Marcel and Michael. It's low-level, which makes it very fast and easy to install and embed in other languages. In particular, its Node.js bindings are a very popular way to use Sass in the JavaScript world.

Each implementation's strengths complement the other's weaknesses. Where LibSass is fast and portable, Ruby Sass is slow and difficult for non-Ruby-users to install. Where Ruby Sass is easy to iterate on, LibSass's low-level language makes it substantially harder to add new features. A complementary relationship can be healthy, but it can also mean that neither solution is as good as it needs to be. That's what we found when, in May, Marcel officially left the LibSass team1.

Without two people's worth of effort, we were no longer sure that LibSass could keep pace with the speed Chris and I wanted to introduce changes into the language. And it had been clear for a long time that Ruby Sass was far too slow for use cases involving large stylesheets. We needed a new implementation, one that could generate CSS quickly and add new features quickly.

Why Dart?

We considered a number of possible languages, and ended up deciding on Dart for a number of reasons. First, it's really fast—the Dart VM is generally much faster than JavaScript VMs, and early benchmarks2 indicate that, for large stylesheets, Dart Sass is 5-10x faster than Ruby Sass and only about 1.5x slower than LibSass. I'll hazard a guess that it would be about 1.5-2x faster than an idiomatic JS implementation, but I can't say for sure. And Dart's performance continues to get better all the time.

At the same time, Dart is easy to work with—much more so than C++, and to some extent even more than Ruby for such a large project. Granted, not as many people are familiar with it as with JavaScript, but language implementations don't tend to get many external contributions anyway. I'll be doing most of the work on the new implementation, and Dart is the language that I'm personally most comfortable with at the moment (when I'm not working on Sass, I'm on the Dart team). Using Dart gives me a lot of extra velocity.

Unlike Ruby or JavaScript, Dart is statically typed, so every value's type can be figured out without running the code. Unlike C++, it's garbage collected, so we don't have to worry as much about cleaning up after ourselves. This makes it easy to write, easy to modify, and easy to maintain. Maybe even more importantly, it makes it easy to translate to other programming languages, which will help LibSass get new features faster.

The last reason we chose Dart is something that only a few other languages can boast: JavaScript compatibility. Dart can be compiled to JavaScript, which can be used directly in Node.js or even potentially run in a browser. A huge chunk of the Sass ecosystem built on node-sass, and we intend to make the JS version of Dart Sass as close to API-compatible with node-sass as possible, so that it can easily drop into existing tools and build systems.

The only downside is that there's a speed hit: Dart Sass is about twice as slow running on V8 as it is running on the Dart VM. However, this still puts it solidly 3-4x faster than Ruby Sass. Ultimately we also hope to provide an easy path for users of the JS-compiled version to move to the Dart VM version as little friction as possible.

What Will Happen to The Other Implementations?

Nothing's changing about LibSass's development. Michael's hard at work adding features from Sass 3.5, and we expect that process to continue as new language features are added. The only difference is that LibSass will no longer be required to be strictly compatible with the latest version of the language in order for that version to launch, since it will no longer be the only implementation with reasonable performance.

More flexibility translates into faster LibSass releases that prioritize the features users want most. Strict compatibility meant that important features like CSS custom property support can't be released until all the tiny tricky edge cases that were in the corresponding Ruby Sass release, like :root unification, are implemented as well. We'll still strive for as much compatibility as possible, but we won't let that stand in the way of velocity.

Ruby Sass, on the other hand, will eventually go away unless a new maintainer appears. We don't want to make the transition sudden and risk fracturing the ecosystem: Chris and I are committed to maintaining it for one year, which includes keeping the language in sync with any new additions in Dart Sass. If anyone is interested in volunteering as a maintainer after that period, we'd be thrilled to mentor them and teach them the codebase over the coming year. But if no one steps up, Ruby Sass will be officially considered deprecated and unmaintained.

I want to emphasize that we aren't making the decision to stop developing Ruby Sass lightly. This is a big change, and it's not an easy one for me—I've worked on Ruby Sass continuously for almost ten years now, and it's difficult to let that history go. But Chris and I have discussed this thoroughly, and we're convinced this is the right move. We only have so much time to devote to Sass, and it no longer makes sense to put that time into an implementation that's so slow as to be infeasible for many of our largest users.

What Next?

Before we release the first stable version of Dart Sass, there are a few big things on our to-do list:

  • Full sass-spec compatibility. There are still a bunch of corners of the language where Dart Sass does the wrong thing, especially with respect to @extend. I don't expect any individual incompatibility to be especially difficult to address, and sass-spec is pretty comprehensive, so it's just a matter of steadily reducing the number of failing specs until it hits zero.

  • Close-enough node-sass render() compatibility in the npm package. The node-sass render() API is the main entrypoint to LibSass in the JavaScript world. It's how build systems run Sass, how users define custom Sass functions, and how Eyeglass passes modules to Sass. We want to support this API with enough fidelity that the existing ecosystem works with JS-compiled Dart Sass.

  • Dart Sass compatibility in Ruby Sass. There are some cases where Dart Sass intentionally differs from Ruby Sass, particularly when Ruby Sass's behavior is considered a bug. We should add deprecation messages in Ruby Sass and, if we can do so with minimal disruption, add support for the new behavior.

There's plenty more we'd like to do eventually, like supporting Sass in the browser and providing a node-sass-compatible wrapper for Sass on the Dart VM, but those aren't blocking the initial release.

Onward Into the Future

The next couple months will see a lot of work go into getting Dart Sass stable and compatible, and getting Sass 3.5 features into LibSass. I think it's likely that early 2017 will see a stable release of Dart Sass and a 3.5 release of LibSass. At that point we'll set our sight on the big features and start working towards Sass 4.0 and its brand new module system.

Dart Sass is a big change, but it's an exciting one as well. It'll allow us to get new features into users' hands faster, and to make those features run faster. It'll make it possible for users to trivially install and run the reference implementation. And it'll give us a performant way to run Sass in pure JavaScript Sass for the first time ever. The benefits are large and tangible, and I'm confident they're worth the costs.


  1. I say "officially" because he's still contributing to the project when he can, just not in an official maintainer capacity. 

  2. Caveats apply: I'm not a benchmarking expert, and these tests were ad hoc and run against non-representative source stylesheets. If anyone is interested in working on more scientific benchmarks, please let me know!