Jake Trent

Render CodeMirror on the Server

CodeMirror is a great in-browser editor that needs some help when rendered server-side.

In-browser Code Editor

CodeMirror is a library that provides code and a UI for creating a coding environment inside your browser. There’s an editable text area where you can type code, and as you write it, it’s syntax highlighted just like your editor at home. It’s a cool tool for creating some cool products.

codemirror example

This user interaction happens in the browser, but if your code goes through a server render process, you’re going to need a little help.

Because CodeMirror runs in the browser, it is written to rely on certain browser APIs. One such API it relies on is navigator.

If you just import and use CodeMirror in certain ways in a Node environment, you’ll see something like this:

[0] ERROR in ReferenceError: navigator is not defined
[0]     at evalmachine.<anonymous>:8017:17
[0]     at userAgent (evalmachine.<anonymous>:8010:27)

Conditionally Import Modes

To get around this, you want to conditionally import your “mode”. In CodeMirror, a “mode” is the syntax mode that you’re highlighting – eg, js or css.

In my code, I’m using react-codemirror to render the editor, and users are writing JavaScript in this editor window.

It turns out react-codemirror is fine on the server side, but the JavaScript codemirror mode is not. So I can es6 static import the react-codemirror bits, but I’m going to conditionally load the mode.

(Note that if you’re not using react-codemirror but raw codemirror for the editor code, you’ll need to conditionally import it as well, for the same reasons.)

Anyway, the example:

import 'codemirror/lib/codemirror.css'
import './codemirror-theme-monokai-sublime.css'
import CodeMirror from 'react-codemirror'
import React from 'react'

let modeLoaded = false
if (typeof window !== 'undefined' && typeof window.navigator !== 'undefined') {
  modeLoaded = true

export default props => {
  const options = {
    theme: 'monokai-sublime'
  if (modeLoaded) options.mode = 'javascript'

  return (

Do you see how modeLoaded is protecting CodeMirror from engaging its syntax highlighting on the server?

What other strategies do you use to make CodeMirror work better in a server-rendered environment?