Marijn Haverbeke's blog (license)

CodeMirror 6 Status Update

Friday, August 30, 2019 javascript codemirror

It has been almost a year since we officially announced the CodeMirror 6 project, which aims to rewrite CodeMirror (a code editor for in the browser) to align its design with the technological realities and fashions of the late 2010s.

This post is for you if, in the course of that year, you've occasionally wondered what the hell Marijn is doing and if he's getting anywhere at all. I have been absolutely terrible about communicating progress, and even monitoring the repository would often leave you in the dark, as I was working on local branches or entirely different repositories. In any case, the code that's in there is almost entirely undocumented.

Where We Are

Last week, I landed a major set of changes, which had been in the works for about four months. They integrate a new approach to code parsing. This was the last piece of the system that was completely in flux, where I didn't want to nail down anything related to it yet because I wasn't sure how it would end up working.

Now I am sure, which means that the vision for the system as a whole just became a lot more clear.

Apart from the parser work, a lot of design and cleanup has happened in the past year. I'll go over the major elements at the end of this post.

For observers, the more interesting thing is that we finished a doc comment extractor for TypeScript, and are starting to work on adding enough comments to generating a passable reference manual for the new system. Hopefully, that will finally allow outsiders to get a clear view of what we're building.

I intend to start releasing npm packages around that time. They won't be stable or feature-complete. They won't even reflect the package organization of the final system. But they should provide an easy way to start playing with the system.

Where We Are Not

I'm a bit behind where I had hoped I would be at this point. This has three reasons:

So the bad news is that a stable release is still quite a ways off (not going to make any more specific predictions at this point). Some parts of the system are at a point where they may roughly stay the way they are, but other significant parts haven't even been written yet.

What We Did

These are the major pieces of work that have happened since the project was announced...

Composition Support

Handling composition/IME (input method editor, as used by people whose script has too many characters to fit on a keyboard, but also pretty much all Android virtual keyboard input) in the browser is its own special kind of hell.

During composition, the user interface is in a special mode where a piece of the editable text is being composed. What exactly that means differs per input method, but it usually involves step-by-step updates through pressing additional keys and/or interacting with menus to reach the intended input string.

In normal operation, CodeMirror will constantly sync the content in the DOM with its model of that content—applying highlighting and normalizing DOM elements that don't have the expected shape. However, during composition, if you mess with the DOM around the composition, or with the selection, you will abort the composition, making your editor pretty much unusable to IME users and, on some browsers, causing bizarre side effects such as their composed text being duplicated again and again on every keystroke.

Our old approach was to freeze interface updates during composition, stepping well back and letting the user do their thing until the composition ended, at which point the editor sprang to life again. That led to a number of responsiveness issues, where editor plugins, such as autocomplete, wouldn't be kept up to date on what is happening, causing the interface to appear frozen or outdated.

In CodeMirror 6, I implemented a subtle middle road where the part of the document that is being composed is left alone as long as its content isn't changed by outside code, but the editor's update cycle proceeds as normal, and the part of the document outside of the composition can be changed (say, by collaborative editing, or by the autocompletion plugin updating its list of completions) as normal.

Behaviors as Extension Primitive

A design where a generic core provides a platform for all kinds of features to be implemented in plugins has to, in a way, provide an extendable extension mechanism, where extensions can define new ways in which other extensions can add or configure behavior. For example, an autocompletion plugin must be able to provide a way for other plugins to register completion sources.

Designing this is tricky, but I think we landed on a nice architecture. I've written another blog post outlining our design. This is working well so far, and has allowed us to create all kinds of features (from the undo history to syntax highlighting to line number gutters) outside of the core library.

CSS Modules

Since the browser platform's CSS support is still more or less completely unhelpful when it comes to modularized system, we've decided to use a CSS-in-JS approach where extensions can define their own styles from within JavaScript code and make sure they are included in the document or shadow DOM where the editor is placed.

View Fields and Updates

The place where pure code (around the immutable editor state abstraction) stopped and imperative code (around the DOM and the editor view) started hadn't really been properly defined in the first iteration.

A pain point was viewport state—information about which part of the document is currently visible. This isn't part of the core editor state, but many UI extensions need to access it in order to operate, usually because, as an optimization, they only act on the visible code.

I've added a new abstraction, view fields, for pieces of state that live in the view and affect things like decorations (styling and widgets in the content) and the attributes of the editor's wrapper DOM nodes.

These can be written in bog-standard imperative style, if you want, but still make it easy to handle editor state changes in a disciplined way—they are notified each time something changes, and provided with a full description of the update, including the viewport information and the transactions that were applied.

Block Widgets

Block widgets are a way to insert a block element into the document, either on a line boundary, in the middle of a line, or covering some content.

These existed before but I completely reimplemented them (and the rest of the decoration system) at the start of the year to fix some corner case issues in the old implementation, cleaning up a number of issues and limitations in the display code.

Interestingly, allowing block widgets in the middle of lines, which wasn't initially part of the plan, turned out to be easier than forbidding them, due to the way decorations interact. Another decoration could hide the newline next to a block widget, something the initial implementation could not deal with gracefully.

Generic Gutters

The first demo came with a line number gutter, but no way to create gutters for other purposes. I generalized the gutter plugin so that is now possible to create your own custom gutters and dynamically add or change the content that is displayed in them.

Doc Generation

We wrote a system that uses the TypeScript library to extract both doc comments and types from a TypeScript project, and output them as a JSON structure. Feeding this into the builddocs tool allows us to build good-looking, interlinked reference documentation directly from the source code.

The system is already being used to generate Lezer's reference docs, and we're working on applying it to CodeMirror.


This is the biggest one, and kept me busy for most of the spring and summer. It was clear that we'd want to move beyond CodeMirror 5's primitive syntax analysis system, but it wasn't clear how.

That is, until I saw tree-sitter, which is a practical implementation of an incremental LR parser, giving you a real syntax tree for your code and updating it cheaply when you make changes. It is used in the Atom editor.

Unfortunately, tree-sitter is written in C, which is awkward to run in the browser (we're still targeting non-WASM browsers). It also generates very hefty grammar files because it makes the size/speed trade-off in a different way than a browser-based system would.

Thus, I set out to clone tree-sitter in JavaScript. And because I always feel I know everything better, I didn't exactly clone it, but rather built a different system inspired by it. That system is Lezer, an incremental parser generator and runtime system for JavaScript.

This was a relatively big project. And then, when I started integrating it with CodeMirror 6, I was forced to go back to the drawing board several times to work out issues.

But the system turned out well. If a grammar has been written for your language (we have JS, CSS, HTML, and XML so far), CodeMirror can keep a syntax tree of your document as you are editing it. This tree is used for (more accurate) highlighting, indentation, folding (soon) and has a host of other potential uses.