Example: Configuration

A CodeMirror editor's configuration lives in its state object. When creating a state, you pass it a set of extensions to use, which will be resolved into an effective configuration.

Extensions can be created by various library functions, and do things like adding an input for a facet or installing a state field. They can be grouped in arrays, and most practical extensions consist of multiple smaller extensions. See this page for a list of extensions provided by core libraries.

For example, the history extension contains a state field that records the undo history, a facet that controls the extension's configuration, and a view plugin that listens for beforeinput events. Code using the history doesn't have to worry about that though—it can just drop the extension value produced by the function into its configuration to install all the necessary parts.

The basic setup is a larger example of this—it holds an array throwing together a whole cartload of different extensions, which together configure a basic code editor.


With facet inputs, order matters. The configuration resolves them in a specific order, and for most facets, that order matters. For example with event handlers, transaction filters, or keymaps, it determines which gets called first. With settings like the line separator or indentation unit, the value with the highest precedence wins.

When a configuration is resolved, by default, the tree of nested arrays of extensions are simply flattened into a sequence. Inputs to a given facet are then collected in the order they appear in that sequence.

So if you specify [keymap.of(A), keymap.of(B)] as a configuration, bindings from A take precedence over those in B. If you nest more deeply...

[..., [keymap.of(A)], ..., [[], ..., [keymap.of(B)]]]

... A still comes before B in the flat ordering.

There is just one further complication, which allows extensions to provide a hint on where sub-extensions should go. When an extension is wrapped in a call to one of the properties of Prec, its parts are put in a different bucket from the default sequence during configuration flattening. There are highest, high, default, low, and lowest buckets.

Within each bucket, ordering is still controlled by the position of the extensions in the flattened sequence. But all extensions in a higher-precedence bucket come before all extensions in a lower-precedence one.

So in an extension like this, A

[Prec.high(A), B, Prec.high([C, D])]

The extensions will be ordered A, C, D, B, because the ones with high precedence come before B with its default precedence.

In general, explicit precedence is used by extension providers to put (parts of) their extension in a specific bucket. For example, a special-purpose key binding that only applies in a specific situation might be given an extending precedence, so that it gets to run before the bulk of the keybindings. Or a default piece of configuration might be given a fallback precedence so that any other values provided will by default override it.

Ordering of extensions in arrays, on the other hand, is generally done by the code putting together a complete configuration from various extension providers, and allows for a more global control.

Dynamic Configuration

In this configuration system you can't just call a set-option method to change your configuration on the fly, because configuration doesn't have the shape that it has in, for example, CodeMirror 5: A map from option names to values.

That is mostly a good thing—extensions will need to affect configuration, and doing that without stepping on each other's toes is difficult when each option has exactly one value, which is replaced with a new value when updated. Facets take multiple inputs and explicitly define some way to combine them, which mostly avoids such conflicts.


Still, dynamic reconfiguration is useful in an editor. In order to be able to partially reconfigure a tree of extensions, we need to divide it into compartments. Transactions can update the configuration by replacing the content of individual compartments.

import {basicSetup, EditorView} from "codemirror"
import {EditorState, Compartment} from "@codemirror/state"
import {python} from "@codemirror/lang-python"

let language = new Compartment, tabSize = new Compartment

let state = EditorState.create({
  extensions: [

let view = new EditorView({
  parent: document.body

When you've done this, you can dispatch transactions to change your configuration.

function setTabSize(view, size) {
    effects: tabSize.reconfigure(EditorState.tabSize.of(size))

Private Compartments

The above example shows how the editor's main configuration can be divided into compartments. But compartments may nest, and the extension tree produced by some plugin may use its own compartments to dynamically en- or disable some extensions.

If you just need to dynamically derive the value of some facet from other aspects of the state, it is preferable to use computed facets instead of reconfiguration, since those are more efficient and easier to keep track of (they are a form of derived state, rather than adding new fundamental state).

But if you have something like an extension that wants to conditionally enable another extension, locally declaring a compartment and reconfiguring that as needed works well.

For example, this function returns an extension that binds the given key to toggle another extension on and off.

import {Extension, Compartment} from "@codemirror/state"
import {keymap, EditorView} from "@codemirror/view"

export function toggleWith(key: string, extension: Extension) {
  let myCompartment = new Compartment
  function toggle(view: EditorView) {
    let on = myCompartment.get(view.state) == extension
      effects: myCompartment.reconfigure(on ? [] : extension)
    return true
  return [
    keymap.of([{key, run: toggle}])

With which you can do something like...

toggleWith("Mod-o", EditorView.editorAttributes.of({
  style: "background: yellow"

If the parent compartment of such an extension is reconfigured, the extension, along with its local compartment, will simply vanish from the configuration.

Top-Level Reconfiguration

Sometimes you need to replace the system's main configuration. There's a state effect that replaces the top-level extension that was provided when creating the state with a new one.

import {StateEffect} from "@codemirror/state"

export function deconfigure(view) {
    effects: StateEffect.reconfigure.of([])

That function is never a good idea, since it'll mostly just render your editor useless, but it shows how to do a top-level reconfiguration. This is slightly different from just creating a new editor state, in that it'll preserve the content of state fields and compartments that exist in both the old and the new configuration.

Another thing you can do is add extension with the appendConfig effect. Extensions added in this way are added to the end of the top-level configuration, and stay there until a full reconfiguration happens.

This can be useful to inject extensions on demand. For example, snippet completion, the first time it is activated, adds a state field that tracks which snippet field the user is in.

function injectExtension(view, extension) {
    effects: StateEffect.appendConfig.of(extension)

Automatic Language Detection

This example sets up an editor that dynamically changes its language configuration in response to the (auto-detected) language of the editor content.

In order to be able to affect transactions as they are being created (as opposed to dispatching a separate reconfiguring extension after the change), we'll use a transaction extender. Whenever the document content changes, our extender does a crude check (whether the doc starts with a < character) to determine whether the document contains HTML or JavaScript code.

When the detected language disagrees with the (primary) language configured for the state, the transaction is extended with a reconfiguration effect that switches the language config compartment to the appropriate extensions.

import {EditorState, Compartment} from "@codemirror/state"
import {htmlLanguage, html} from "@codemirror/lang-html"
import {language} from "@codemirror/language"
import {javascript} from "@codemirror/lang-javascript"

const languageConf = new Compartment

const autoLanguage = EditorState.transactionExtender.of(tr => {
  if (!tr.docChanged) return null
  let docIsHTML = /^\s*</.test(tr.newDoc.sliceString(0, 100))
  let stateIsHTML = tr.startState.facet(language) == htmlLanguage
  if (docIsHTML == stateIsHTML) return null
  return {
    effects: languageConf.reconfigure(docIsHTML ? html() : javascript())

If we specify an initial language configuration, we must be careful to wrap it with our compartment, so that when the extension updates the language, that part of the configuration gets replaced.

import {EditorView, basicSetup} from "codemirror"

new EditorView({
  doc: 'console.log("hello")',
  extensions: [
  parent: document.querySelector("#editor")

The result acts like this: