System Guide

This is the guide to the CodeMirror editor system. It provides a prose description of the system's functionality. For the item-by-item documentation of its interface, see the reference manual.

Architecture Overview

Because CodeMirror is structured quite a bit differently than your classical JavaScript library (including its own previous versions), it is recommended to read at least this section before jumping in, so that you don't waste your time with mismatched expectations.

Modularity

CodeMirror is set up as a collection of separate modules that, together, provide a full-featured text and code editor. On the bright side, this means that you can pick and choose which features you need, and even replace core functionality with a custom implementation if you need to. On the less bright side, this means that setting up an editor requires you to put together a bunch of pieces.

The putting-together part isn't hard, but you will have to install and import the pieces you need. The core packages, without which it'd be hard to set up an editor at all, are:

This is what a minimal viable editor might look like:

import {EditorState} from "@codemirror/state"
import {EditorView, keymap} from "@codemirror/view"
import {defaultKeymap} from "@codemirror/commands"

let startState = EditorState.create({
  doc: "Hello World",
  extensions: [keymap.of(defaultKeymap)]
})

let view = new EditorView({
  state: startState,
  parent: document.body
})

Many things that you might expect to be part of the core, such as the line number gutter or undo history are in fact separate packages. To make it easy to get started, the @codemirror/basic-setup package pulls in most of the things you need for a baseline editor (except a language package).

import {EditorState, EditorView, basicSetup} from "@codemirror/basic-setup"
import {javascript} from "@codemirror/lang-javascript"

let view = new EditorView({
  state: EditorState.create({extensions: [basicSetup, javascript()]}),
  parent: document.body
})

The packages are distributed as ES6 modules. This means that it is not currently practical to run the library without some kind of bundler (which packages up a modular program into a single big JavaScript file) or module loader. If you are new to bundling, I recommend looking into rollup or Webpack.

Functional Core, Imperative Shell

An attitude that guides the architecture of CodeMirror is that functional (pure) code, which creates new values instead of having side effects, is much easier to work with than imperative code. But the browser DOM is obviously very imperative-minded, as are many of the systems that CodeMirror integrate with.

To resolve this contradiction, the library's state representation is strictly functional—the document and state data structures are immutable, and operations on them are pure functions, whereas the view component and command interface wrap these in an imperative interface.

This means that an old state value stays intact even when the editor moves on to a new state. Having both the old and the new state available is often very useful when dealing with state changes. It also means that directly changing a state value, or writing extensions like additional state fields in an imperative way will not do what you'd hope (and probably just break things).

The TypeScript interface tries to be very clear about this by marking arrays and object properties as readonly. When using plain old JavaScript it can be harder to remember this. But as a general rule, unless explicitly described in the docs, reassignment of properties in objects created by the library is just not supported.

let state = EditorState.create({doc: "123"})
// BAD WRONG NO GOOD CODE:
state.doc = Text.of("abc") // <- DON'T DO THIS

State and Updates

The library handles updates in a way inspired by approaches like Redux or Elm. With a few exceptions (like composition and drag-drop handling), the state of the view is entirely determined by the EditorState value in its state property.

Changes to that state happen in functional code, by creating a transaction that describes the changes to document, selection, or other state fields. Such a transaction can then be dispatched, which tells the view to update its state, at which point it'll synchronize its DOM representation with the new state.

// (Assume view is an EditorView instance holding the document "123".)
let transaction = view.state.update({changes: {from: 0, insert: "0"}})
console.log(transaction.state.doc.toString()) // "0123"
// At this point the view still shows the old state.
view.dispatch(transaction)
// And now it shows the new state.

The data flow during typical user interaction looks something like this:

DOM event
view
transaction
new state

The view listens for events. When DOM events come in, it (or a command bound to a key, or an event handler registered by an extension) translates them into state transactions and dispatches them. This builds up a new state. When that new state is given to the view, it'll update itself.

Extension

Since the core library is rather minimal and generic, a lot of functionality is implemented in system extensions. Extensions can do all kinds of things, from merely configuring some option, to defining new fields in the state object, to styling the editor, to injecting custom imperative components into the view. The system takes care to allow extensions to compose without unexpected conflicts.

The set of active extensions is kept in the editor state (and can be changed by a transaction). Extensions are provided as values (usually imported from some package), or arrays of such values. They can be arbitrarily nested (an array containing more arrays is also a valid extension), and are deduplicated during the configuration process. Thus, it is okay for extensions to pull in other extensions—if the same one gets included multiple times, it'll only take effect once.

When relevant, the precedence of extensions is determined first by explicitly set precedence category, and within that, by the position the extension has in the (flattened) collection of extensions passed to the state.

import {keymap} from "@codemirror/view"
import {EditorState, Prec} from "@codemirrror/state"

function dummyKeymap(tag) {
  return keymap.of([{
    key: "Ctrl-Space",
    run() { console.log(tag); return true }
  }])
}

let state = EditorState.create({extensions: [
  dummyKeymap("A"),
  dummyKeymap("B"),
  Prec.override(dummyKeymap("C"))
]})

In a view using the state from that code, pressing Ctrl-Space will log "C", because, despite being last in the order of extensions, that keymap is tagged with a higher precedence. If that weren't the case, keymap "A" would be the first one to get a chance to handle the key combination, since it occurs before the others.

A later section of the guide goes into more detail on the kinds of extensions that exist, and how to use them.

Document Offsets

CodeMirror uses plain numbers to address positions in the document. These represent character counts—more precisely, they count UTF16 code units (so astral characters count as two units). Line breaks always count as a single unit (even when you configure a line separator that is longer than that).

These offsets are used to track the selection, position changes, decorate content, and so on.

It is sometimes necessary to figure out where a position in a start document ends up in a changed document. For this purpose, the library provides a position mapping feature, which, given a transaction (or just a change set) and a start position, can give you the corresponding new position.

import {EditorState} from "@codemirror/state"

let state = EditorState.create({doc: "1234"})
// Delete "23" and insert at "0" at the start.
let tr = state.update({changes: [{from: 1, to: 3}, {from: 0, insert: "0"}]})
// The position at the end of the old document is at 3 in the new document.
console.log(tr.changes.mapPos(4))

The document data structure also indexes by lines, so it is not expensive to look things up by (1-based) line number.

import {Text} from "@codemirror/text"

let doc = Text.of(["line 1", "line 2", "line 3"])
// Get information about line 2
console.log(doc.line(2)) // {start: 7, end: 13, ...}
// Get the line around position 15
console.log(doc.lineAt(15)) // {start: 14, end: 20, ...}

Data Model

CodeMirror, being a text editor, treats the document as a flat string. It stores this in a tree-shaped data structure to allow cheap updates anywhere in the document (and efficient indexing by line number).

Document Changes

Document changes are themselves values, describing precisely which ranges of the old document are being replaced by which bits of new text. This allows extensions to track precisely what happens to the document, allowing things like an undo history and collaborative editing to be implemented outside the library core.

When creating a change set, all changes are described in terms of the original document—they conceptually all happen at once. (If you really need to combine lists of changes where later changes refer to the document created by earlier ones, you can use the change set compose method.)

Selection

Alongside the document, an editor state stores a current selection. Selections may consist of multiple ranges, each of which can be a cursor (empty) or cover a range between its anchor and head). Overlapping ranges are automatically merged, and ranges are sorted, so that a selection's ranges property always holds a sorted, non-overlapping array of ranges.

import {EditorState, EditorSelection} from "@codemirror/state"

let state = EditorState.create({
  doc: "hello",
  selection: EditorSelection.create([
    EditorSelection.range(0, 4),
    EditorSelection.cursor(5)
  ]),
  extensions: EditorState.allowMultipleSelections.of(true)
})
console.log(state.selection.ranges.length) // 2

let tr = state.update(state.replaceSelection("!"))
console.log(tr.state.doc.toString()) // "!o!"

One of these ranges is marked as the main one. This is the one that the browser's DOM selection will reflect. The others are drawn and handled entirely by the library.

By default a state will only accept selections with a single range. You have to enable an extension (like drawSelection) that enables multiple selection ranges to get access to them.

State objects have a convenience method, changeByRange for applying an operation to every selection range separately (which can be a bit awkward to do manually).

import {EditorState} from "@codemirror/state"

let state = EditorState.create({doc: "abcd", selection: {anchor: 1, head: 3}})

// Upcase the selection
let tr = state.update(state.changeByRange(range => ({
  changes: {from: range.from, to: range.to,
            insert: state.sliceDoc(range.from, range.to).toUpperCase()},
  // The updated selection range—in this case it stays the same
  range
})))
console.log(tr.state.doc.toString()) // "aBCd"

Configuration

Each editor state also has a (private) reference to its configuration, which is determined by the extensions that are active for that state. During regular transactions, the configuration stays the same. But it is possible to reconfigure the state using the reconfigure transaction directive.

The main effects of a state's configuration are the fields it stores, and the values associated with facets for that state.

Facets

A facet is an extension point. Different extension values can provide values for the facet. And anyone with access to the state and the facet can read its combined value. Depending on the facet, that may just be an array of provided values, or some value computed from those.

The idea behind facets is that most types of extension allow multiple inputs, but want to compute some coherent combined value from those. How that combining works may differ.

import {EditorState} from "@codemirror/state"

let state = EditorState.create({
  extensions: [
    EditorState.tabSize.of(16),
    EditorState.changeFilter.of(() => true)
  ]
})
console.log(state.facet(EditorState.tabSize)) // 16
console.log(state.facet(EditorState.changeFilter)) // [() => true]

Facets are explicitly defined, producing a facet value. Such a value can be exported, to allow other code to provide and read it, or it can be kept module-private, in which case only that module can access it. We'll come back to that in the section on writing extensions.

In a given configuration, most facets tend to be static, provided only directly as part of the configuration. But it is also possible to have facet values computed from other aspects of the state.

Facet values are only recomputed when necessary, so you can use an object or array identity test to cheaply check whether a facet changed.

Transactions

Transactions, created with the state's update method, combine a number of effect (all optional):

To completely reset a state—for example to load a new document—it is recommended to create a new state instead of a transaction. That will make sure no unwanted state (such as undo history events) sticks around.

The View

The view tries to be as transparent a layer around the state as possible. Unfortunately, there are some aspects of working with an editor that can't be handled purely with the data in the state.

The library does not expect user code to manipulate the DOM structure it manages. When you do try that, you'll probably just see the library revert your changes right away. See the section on decorations for the proper way to affect the way the content is displayed.

Viewport

One thing to be aware of is that CodeMirror doesn't render the entire document, when that document is big. To avoid doing some work, it will, when updating, detect which part of the content is currently visible (not scrolled out of view), and only render that plus a margin around it. This is called the viewport.

Querying coordinates for positions outside of the current viewport will not work (since they are not rendered, and thus have no layout). The view does track height information (initially estimated, measured accurately when content is drawn) for the entire document, even the parts outside of the viewport.

Long lines (when not wrapped) or chunks of folded code can still make the viewport rather huge. The editor also provides a list of visible ranges, which won't include such invisible content. This can be useful when, for example, highlighting code, where you don't want to do work for text that isn't visible anyway.

Update Cycle

CodeMirror's view makes a serious effort to minimize the amount of DOM reflows it causes. Dispatching a transaction will generally only cause the editor to write to the DOM, without reading layout information. The reading (to check whether the viewport is still valid, whether the cursor needs to be scrolled into view, and so on) is done in a separate measure phase, scheduled using requestAnimationFrame. This phase will, if necessary, follow up with another write phase.

You can schedule your own measure code using the requestMeasure method.

To avoid weird reentrancy situations, the view will raise an error when a new update is initiated while another update is in the process of being (synchronously) applied. Multiple updates applied while a measure phase is still pending are not a problem—those will just cause their measure phases to be combined.

When you are done with a view instance, you can call its destroy method to dispose of it.

DOM Structure

The editor's DOM structure looks something like this:

<div class="cm-editor [theme scope classes]">
  <div class="cm-scroller">
    <div class="cm-content" contenteditable="true">
      <div class="cm-line">Content goes here</div>
      <div class="cm-line">...</div>
    </div>
  </div>
</div>

The outer (wrap) element is a vertical flexbox. Things like panels and tooltips can be put here by extensions.

Inside of that is the scroller element. If the editor has its own scrollbar, this one should be styled with overflow: auto. But it doesn't have to—the editor also supports growing to accomodate its content, or growing up to a certain max-height and then scrolling.

The scroller is a horizontal flexbox element. When there are gutters, they are added to its start.

Inside that is the content element, which is editable. This has a DOM mutation observer registered on it, and any changes made in there will result in the editor parsing them as document changes and redrawing the affected nodes. This container holds a line element for each line in the viewport, which in turn hold the document text (possibly decorated with styles or widgets).

Styles and Themes

To manage editor-related styles, CodeMirror uses a system to inject styles from JavaScript. Styles can be registered with a facet, which will cause the view to make sure they are available.

Many elements in the editor are assigned classes prefixed with cm-, as generated by the themeClass function. These can be styled directly in your local CSS. But they can also be targeted by themes. A theme is an extension created with EditorView.theme. It gets its own unique (generated) CSS class (which will be added to the editor when the theme extension is active) and defines styles scoped by that class.

A theme declaration defines any number of CSS rules using style-mod notation. The editor uses class names prefixed by "cm-". This code creates a crude theme that makes the default text color in the editor orange:

import {EditorView} from "@codemirror/view"

let view = new EditorView({
  extensions: EditorView.theme({
    ".cm-content": {color: "darkorange"},
    "&.cm-focused .cm-content": {color: "orange"}
  })
})

To allow the automatic class prefixing be done correctly, rules where the first element targets the editor's wrapper element (which is the where the theme's unique class will be added), such as the .cm-focused rule in the example, must use an & character to indicate the position of the wrapper element.

Extensions can define base themes to provide default styles for the elements they create. Base themes can use &light (default) and &dark (enabled when there's a dark theme active) placholders, so that even when they aren't overridden by a theme, they don't look too out of place.

import {EditorView} from "@codemirror/view"

// This again produces an extension value
let myBaseTheme = EditorView.baseTheme({
  "&dark .cm-mySelector": { background: "dimgrey" },
  "&light .cm-mySelector": { background: "ghostwhite" }
})

Commands

Commands are functions with a specific signature. Their main use is key bindings, but they could also be used for things like menu items or command palettes. A command function represents a user action. It takes a view and returns a boolean, false to indicate it doesn't apply in the current situation, and true to indicate that it successfully executed. The effect of the command is produced imperatively, usually by dispatching a transaction.

When multiple commands are bound to a given key, they are tried one at a time until one of them returns true.

Commands that only act on the state, not the entire view, can use the StateCommand type instead, which is a subtype of Command that just expects its argument to have state and dispatch properties. This is mostly useful for being able to test such commands without creating a view.

Extending CodeMirror

There are a number of different ways to extend CodeMirror, and picking the proper way for a given use case isn't always obvious. This section goes over the various concepts you'll need to be familiar with to write editor extensions.

State Fields

Extensions often need to store additional information in the state. The undo history needs to store undoable changes, the code folding extension needs to track what has been folded, and so on.

For this purpose, extensions can define additional state fields. State fields, living inside the purely functional state data structure, must store immutable values.

State fields are kept in sync with the rest of the state using something like a reducer. Every time the state updates, a function is called with the field's current value and the transaction, which should return the field's new value.

import {EditorState, StateField} from "@codemirror/state"

let countDocChanges = StateField.define({
  create() { return 0 },
  update(value, tr) { return tr.docChanged ? value + 1 : value }
})

let state = EditorState.create({extensions: countDocChanges})
state = state.update({changes: {from: 0, insert: "."}}).state
console.log(state.field(countDocChanges)) // 1

You will often want to use annotations or effects to communicate what is happening to your state field.

It can be tempting to try to avoid taking the step of putting state in an actual state field—there's a bit of verbosity involved in declaring one, and firing off a whole transaction for every state change may feel a bit heavyweight. But in almost all cases, it really helps to tie your state into the editor-wide state update cycle, because it makes it a lot easier to keep everything in sync.

Affecting the View

View plugins provide a way for extensions to run an imperative component inside the view. This is useful for things like event handlers, adding and managing DOM elements, and doing things that depend on the current viewport.

This simple plugin displays the document size in the editor's corner.

import {ViewPlugin} from "@codemirror/view"

const docSizePlugin = ViewPlugin.fromClass(class {
  constructor(view) {
    this.dom = view.dom.appendChild(document.createElement("div"))
    this.dom.style.cssText =
      "position: absolute; inset-block-start: 2px; inset-inline-end: 5px"
    this.dom.textContent = view.state.doc.length
  }

  update(update) {
    if (update.docChanged)
      this.dom.textContent = update.state.doc.length
  }

  destroy() { this.dom.remove() }
})

View plugins should generally not hold (non-derived) state. They work best as shallow views over the data kept in the editor state.

When the state is reconfigured, view plugins that aren't part of the new configuration will be destroyed (which is why, if they made changes to the editor, they should define a destroy method that undoes those changes).

When a view plugin crashes, it is automatically disabled to avoid taking down the entire view.

It is possible for view plugins to provide fields, which are a bit like facets on the view level, in that multiple plugins can contribute a given plugin field. This is mostly used for document decorations.

Decorating the Document

When not told otherwise, CodeMirror will draw the document as plain text. Decorations are the mechanism through which extensions can influence what the document looks like. They come in four types:

Decorations can be provided in two ways. There is a state facet that allows you to provide decorations at the level of the editor state, usually in a way derived from a state field. This doesn't allow you to decorate only the viewport (because the state doesn't know about that), so it is mostly useful for things like folded regions or lint annotations.

The other way to decorate is from a view plugin. This is used by features like syntax or search-match highlighting, since view plugins can read the current viewport to avoid doing work for currently-invisible content.

Decorations are kept in sets, which are again immutable data structures. Such sets can be mapped across changes (adjusting the positions of their content to compensate for the change) or rebuilt on updates, depending on the use case.

Extension Architecture

To create a given piece of editor functionality you often need to combine different kinds of extension: a state field to keep state, a base theme to provide styling, a view plugin to manage in- and output, some commands, maybe a facet for configuration.

A common pattern is to export a function that returns the extension values necessary for your feature to work. Making this a function, even if it doesn't (yet) take parameters is a good idea—it makes it possible to add configuration options later, without breaking backwards compatiblity.

Since extensions can pull in other extensions, it can be useful to consider what happens when your extension is included multiple times. For some kinds of extensions, for example keymaps, it is appropriate to just do the thing it's doing multiple times. But often that would be wasteful or even break something.

It is usually possible to make multiple uses of an extension just do the right thing by relying on the deduplication of identical extension values—if you make sure you create your static extension values (themes, state fields, view plugins, etc) only once, and always return the same instance from your extension constructor function, you'll only get one copy of them in the editor.

But when your extension allows configuration, your other logic will probably need access to that. And what do you do when the different instances of the extension got different configurations?

Sometimes, that's just an error. But often, it is possible to define a strategy for reconciling them. Facets work well for this. You can put the configuration in a module-private facet, and have its combining function either reconcile configurations or thow an error when this is impossible. Then code that needs access to the current configuration can read that facet.

See the zebra stripes example for an illustration of this approach.