CodeMirror is a JavaScript component that provides a code editor in the browser.

The first version of the editor was written early 2007, for the console in the Eloquent JavaScript website.

This version was based on the contentEditable feature of browsers.

[2] In late 2010, the Ace project, another JavaScript-based code editor, pioneered new implementation techniques and demonstrated that it is possible, even in JavaScript, to handle documents with many thousands of lines without degraded performance.

The result was version 2, which no longer relied on contentEditable and significantly improved performance.