Introducing a New Thimble and Bramble


This week we're shipping something really cool with Mozilla, and I wanted to pause and tell you about what it is, and how it works.

The tldr; is that we took the Mozilla Foundation's existing web code editor, Thimble, and rewrote it to use Bramble, our forked version of the Brackets editor, which runs in modern web browsers. You can try it now at

If you're the type who prefers animated pictures to words, I made you a bunch over on the wiki, showing what a few of the features look like in action. You can also check out Luke's great intro video.

If you're the type who likes words, the rest of this is for you.


I started working on this project two years ago. While at MozFest 2013 I wrote about an idea I had for a new concept app that merged Thimble and Brackets; at the time I called it Nimble.

I was interested in merging these two apps for a number of reasons. First, I wanted to eliminate the "ceiling" users had when using Thimble, wherein they would graduate beyond its abilities, and be forced to use other tools. In my view, Thimble should be able to grow and expand along with a learner's abilities, and a teacher's needs.

Second, people were asking for lots of new features in Thimble, and I knew from experience that the best code is code you don't have to write. I wanted to leverage the hard work of an existing community that was already focused on building a great web coding platform. Writing a coding environment is a huge challenge, and our team wasn't equipped to take it on by ourselves. Thankfully the Brackets project had already solved this.

On Brackets

Brackets was an easy codebase to get started on, and the community was encouraging and willing to help us with patches, reviews, and questions (I'm especially thankful for @randyedmunds and @busykai).

Brackets is written in an AMD module system, and uses requirejs, react, CodeMirror, LESS, jQuery, Bootstrap, loadash, acorn, tern, etc. One of the things I've loved most about working with the Brackets source is that it uses so much of the the best of the open web. It's ~1.3 million lines of code offer APIs for things things like:

  • code hinting, static analysis, and linting
  • language parsing and tokenizing (html, css, js, xml, less)
  • file system operations
  • editors
  • live DOM diff'ing and in-browser preview
  • swappable servers
  • layout, widgets, and dialogs
  • localization, theming, and preferences
  • extension loading at runtime, with hundreds already written

In short, Brackets isn't an editor so much as a rich platform for coding and designing front-end web pages and apps. Bracket's killer feature is its ability to render a live preview of what's in your editor, including dynamic updates as you type, often without needing to save. The preview even has an awareness of changes to linked files (e.g., external stylesheets and scripts).

Another thing I loved was that Brackets wasn't trying to solve code editing in general: they had a very clear mandate that favoured web development, and front-end web development in particular. HTML, CSS, and JavaScript get elevated status in Brackets, and don't have to fight with every other language for features.

All of these philosophies and features melded perfectly with our goal of making a great learning and teaching tool for web programming.

But what about X?

Obviously there are a ton of code editing tools available. If we start with desktop editors, there are a lot to choose from; but they all suffer from the same problem: you have to download 10s of megs of installer, and then you have to install them, along with a web server, in order to preview your work. Consider what's involved in installing each of these (on OS X):

Thimble, on the other hand, is ~1M (877K for Bramble, the rest for the front-end app). We worked extremely hard to get Brackets (38.5M if you install it) down to something that fits in the size of an average web page. If we changed how Brackets loads more significantly, we could get it smaller yet, but we've chosen to keep existing extensions working. The best part is that there is no install: the level of commitment for a user is the URL.

In addition to desktop editors, there are plenty of popular online options, too:

The list goes on. They are all great, and I use, and recommend them all. Each of these tools has a particular focus, and none of them do exactly what the new Thimble does; specifically, none of them tries to deal with trees of files and folders. We don't need to do what these other tools do, because they already do it well. Instead, we focused on making it possible for users to create a rich and realistic environment for working with arbitrary web site/app structures without needing to install a and run a web server.

From localhost to nohost

I've always been inspired by @jswalden's httpd.js. It was written back before there was node.js, back in a time when it wasn't yet common knowledge that you could do anything in JS. The very first time I saw it I knew that I wanted to find some excuse to make a web server in the browser. With nohost, our in-browser web server, we've done it.

In order to run in a browser, Bramble has to be more than just a code editor; it also has to include a bunch of stuff that would normally be provided by the Brackets Shell (similar to and node.js. This means providing a:

  • web server
  • web browser
  • filesystem

and glue to connect those three. Bracket's uses Chrome's remote debugging protocol and node.js to talk between the editor, browser, and server. This works well, but ties it directly to Chrome.

At first I wasn't sure how we'd deal with this. But then an experimental implementation of the Bracket's LiveDevelopment code landed, which switched away from using Chrome and the remote dev tools protocol to any browser and a WebSocket. Then, in the middle of the docs, we found an offhand comment that someone could probably rewrite it to use an iframe and postMessage...a fantastic idea! So we did.

Making it possible for an arbitrary web site to work in a browser-based environment is a little like Firefox's Save Page... feature. You can't just deal with the HTML alone--you also have to get all the linked assets.

Consider an example web page:

<!DOCTYPE html>  
   <meta charset="utf-8">
    <title>Example Page</title>
    <link rel="stylesheet”
    <img src="images/cat.png">
    <script src="script.js"></script>
      // Call function f in script.js

In this basic web page we have three external resources referenced by URL. The browser needs to be able to request styles/style.css, images/cat.png, and script.js in order to fully render this page. And we're not done yet.

The stylesheet might also reference other stylesheets using @import, or might use other images (e.g., background-image: url(...)).

It gets worse. The script might need to XHR a JSON file from the server in order to do whatever f() requires.

Bramble tries hard to deal with these situations through a combination of static and dynamic rewriting of the URLs. Eventually, if/when all browsers ship it, we could do a lot of this with ServiceWorkers. Until then, we made do with what we already have cross browser.

First, Bramble's nohost server recursively rewrites the HTML, and its linked resources, in order to find relative filesystem paths (images/cat.png) and replace them with Blobs and URL objects that point to cached memory resources read out of the browser filesystem.

Parsing HTML with regex is a non-starter. Luckily browsers have a full parser built in, DOMParser. Once we have an in memory DOM vs. HTML text string, we can accurately querySelectorAll to find things that might contain URLs (img, link, video, iframe, etc., avoiding a due to circular references) and swap those for generated Blob URLs from the filesystem. When we're done, we can extract rewritten HTML text from our live in-memory DOM via documentElement.outerHTML, obtaining something like this:

<!DOCTYPE html>  
   <meta charset="utf-8">
    <title>Example Page</title>
    <link rel="stylesheet”
    <img src="blob:https%3A//">
    <script src="blob:https%3A//"></script>
      // Call function f in script.js

All external resources now use URLs to cached memory resources. This HTML can then be itself turned into a Blob and URL object, and used as the src for our iframe browser (this works everywhere except IE, where you have to document.write the HTML, but can use Blob URLs for everything else).

For CSS we do use regex, looking for url(...) and other places where URLs can lurk. Thankfully there aren't a lot, and it's just a matter of reading the necessary resources from disk, caching to a Blob URL, and replacing the filesystem paths for URLs, before generating a CSS Blob URL that can be used in the HTML.

Despite what everyone tells you about the DOM being slow, the process is really fast. And because we own the filesystem layer, whenever the editor does something like a writeFile(), we can pre-generate a URL for the resource, and maintain a cache of such URLs keyed on filesystem paths for when we need to get them again in the future during a rewrite step. Using this cache we are able to live refresh the browser quite often without causing any noticeable slowdown on the main thread.

As an aside, it would be so nice if we could move the whole thing to a worker and be able to send an HTML string, and get back a URL. Workers can already access IndexedDB, so we could read from the filesystem there, too. This would mean having access to DOMParser (even if we can't touch the main DOM from a worker, being able to parse HTML is still incredibly useful for rewriting, diff'ing, etc).

Finally, we do dynamic substitutions of relative paths for generated Blob URLs at runtime by hijacking XMLHttpRequest and using our postMessage link from the iframe to the editor in order to return response data for a given filename.

And it all works! Sure, there's lots of things we won't ever be able to cope with, from synchronous XHR to various types of DOM manipulation by scripts that reference URLs as strings. But for the general case, it works remarkably well. Try downloading and dragging a zipped web site template from into the editor. Bramble doesn't claim to be able to replace a full, local development environment for every use case; however, it makes it unnecessary in most common cases. It's amazing what the modern web can do via storage, file, drag-and-drop, parser, and worker APIs.

Origin Sandboxing

I talk about Thimble and Bramble as different things, and they are, especially at runtime. Bramble is an embeddable widget with an iframe API, and Thimble hosts it and provides some UI for common operations.

I've put a simple demo of the the Bramble API online for people to try (source is here). Bramble uses, but doesn't own its filesystem; nor does it have any notion of where the files came from or where they are going. It also doesn't have opinions about how the filesystem should be laid out.

This is all done intentionally so that we can isolate the editor and preview from the hosting app, running each on a different domain. We want users to be able to write arbitrary code, execute and store it; but we don't want to mix code for the hosting app and the editor/preview. The hosting app needs to decide on a filesystem layout, get and write the files, and then "boot" Bramble.

I've written previously about how we use MessageChannel to remotely host an IndexedDB backed filesystem in a remote window running on another domain: Thimble owns the filesystem and database and responds to proxied requests to do things via postMessage.

In the case of Thimble, we store data in a Heroku app using postgres on the server. Thimble listens for filesystem events, and then queues and executes file update requests over the network to sync the data upstream. Published projects are written to S3, and we then serve them on a secure domain. Because users can upload files to their filesystem in the editor, it makes it easier to transition to an https:// only web.

When the user starts Thimble, we request a project as a gzipped tarball from the publishing server, then unpack it in a Worker and recreate the filesystem locally. Bramble then "mounts" this local folder and begins working with the local files and folders, with no knowledge of the servers (all data is autosaved, and survives refreshes).


Now that we've got the major pieces in place, I'm interested to see what people will do with both Thimble and Bramble. Because we're in a full browser vs. an "almost-browser" shell, we have access to all the latest toys (for example, WebRTC and the camera). Down the road we could use this for some amazing pair programming setups, so learners and mentors could work with each other directly over the web on the same project.

We can also do interesting things with different storage providers. It would be just as easy to have Bramble talk to Github, Dropbox, or some other cloud storage provider. We intentionally kept Thimble and Bramble separate in order to allow different directions in the future.

Then there's all the possibilities that custom extensions opens up (did I mention that Bramble has dynamic extension loading? because it does!). I'd love to see us use bundles of extensions to enable different sorts of learning activities, student levels, and instructional modes. I'm also really excited to see what kind of new curriculum people will build using all of this.

In the meantime, please try things out, file bugs, chat with us on irc #thimble on moznet and have fun making something cool with just your browser. Even better, teach someone how to do it.

Let me close by giving a big shout out to the amazing students (current and former) who hacked on this with me. You should hire them: Gideon Thomas, Kieran Sedgwick, Kenny Nguyen, Jordan Theriault, Andrew Benner, Klever Loza Vega, Ali Al Dallal, Yoav Gurevich, as well as the following top notch Mozilla folks, who have been amazing to us: Hannah Kane, Luke Pacholski, Pomax, Cassie McDaniel, Ashley Williams, Jon Buckley, and others.