Learn about our RFC process, Open RFC meetings & more.Join in the discussion! »


2.3.0 • Public • Published


This is a wrapper for Ace Editor to provide a 2-panel diffing/merging tool that visualizes differences in two documents and allows users to copy changes from to the other.

It's built on top of the excellent, but appallingly-named google-diff-match-patch library (buuuurn). That lib handles the hard part: the computation of the document diffs. Ace-diff just visualizes that information as line-diffs in the editors.


  • Ace (or Brace) Editor 1.1.8 or later (probably works on older versions, but I haven't confirmed)


Take a look at demos on Ace-diff page. The demos illustrate a few different configurations and styles. Hopefully they'll give you a rough sense of what it does and how it works.


  • Compatible with any Ace/Brace Editor mode or theme
  • Accommodates realtime changes to one or both editors
  • Readonly option for left/right editors
  • Control over how aggressively diffs are combined
  • Option to allow users to copy diffs from one side to the other

How to install

Use npm

npm i ace-diff -S
import AceDiff from 'ace-diff';
// optionally, include CSS, or use your own
import 'ace-diff/dist/ace-diff.min.css';
// Or use the dark mode
import 'ace-diff/dist/ace-diff-dark.min.css';


Grab ace-diff from CDN:

<script src="https://unpkg.com/ace-diff@^2.0.0"></script>
<!-- optionally include CSS, or use your own -->
<link href="https://unpkg.com/ace-diff@^2.0.0/dist/ace-diff.min.css" rel="stylesheet">
<!-- optionally there is also a dark mode CSS -->
<link href="https://unpkg.com/ace-diff@^2.0.0/dist/ace-diff-dark.min.css" rel="stylesheet">


<div class="acediff"></div>


Here's an example of how you'd instantiate AceDiff.

var differ = new AceDiff({
  element: '.acediff',
  left: {
    content: 'your first file content here',
  right: {
    content: 'your second file content here',


Because of the way how ACE is positioned, it's important to have Ace-diff running in some container with specified dimensions (and optionally with a position: relative)

Styling the elements is vitally important: the gutter should retain its width even if the user resizes his or her browser. But honestly, how you go about that is very much up to you: you can provide whatever CSS you want, depending on your scenario.

If you want the ace editor's to change height/width based on a user's browser, I find using flexbox the best option - but hell, if you want to use a <table>, knock yourself out. :)

Take a look at the demos for some ideas. They all use flexbox for the layouts, but include some different styles and class names just so you can see.


You can configure your Ace-diff instance through a number of config settings. This object is what you pass to the constructor, like the JavaScript section above.

Default settings

Here are all the defaults. I'll explain each one in details below. Note: you only need to override whatever you want.

  mode: null,
  theme: null,
  element: null,
  diffGranularity: 'broad',
  showDiffs: true,
  showConnectors: true,
  maxDiffs: 5000,
  left: {
    content: null,
    mode: null,
    theme: null,
    editable: true,
    copyLinkEnabled: true
  right: {
    content: null,
    mode: null,
    theme: null,
    editable: true,
    copyLinkEnabled: true
  classes: {
    diff: 'acediff__diffLine',
    connector: 'acediff__connector',
    newCodeConnectorLinkContent: '&#8594;',
    deletedCodeConnectorLinkContent: '&#8592;',

Diffing settings

  • element (string or element object, required). The element used for Ace-diff
  • mode (string, optional). this is the mode for the Ace Editor, e.g. "ace/mode/javascript". Check out the Ace docs for that. This setting will be applied to both editors. I figured 99.999999% of the time you're going to want the same mode for both of them so you can just set it once here. If you're a mad genius and want to have different modes for each side, (a) whoah man, what's your use-case?, and (b) you can override this setting in one of the settings below. Read on.
  • theme (string, optional). This lets you set the theme for both editors.
  • diffGranularity (string, optional, default: broad). this has two options (specific, and broad). Basically this determines how aggressively AceDiff combines diffs to simplify the interface. I found that often it's a judgement call as to whether multiple diffs on one side should be grouped. This setting provides a little control over it.
  • showDiffs (boolean, optional, default: true). Whether or not the diffs are enabled. This basically turns everything off.
  • showConnectors (boolean, optional, default: true). Whether or not the gutter in the middle show show connectors visualizing where the left and right changes map to one another.
  • maxDiffs (integer, optional, default: 5000). This was added a safety precaution. For really massive files with vast numbers of diffs, it's possible the Ace instances or AceDiff will become too laggy. This simply disables the diffing altogether once you hit a certain number of diffs.
  • left/right. this object contains settings specific to the leftmost editor.
  • left.content / right.content (string, optional, default: null). If you like, when you instantiate AceDiff you can include the content that should appear in the leftmost editor via this property.
  • left.mode / right.mode (string, optional, defaults to whatever you entered in mode). This lets you override the default Ace Editor mode specified in mode.
  • left.theme / right.theme (string, optional, defaults to whatever you entered in theme). This lets you override the default Ace Editor theme specified in theme.
  • left.editable / right.editable (boolean, optional, default: true). Whether the left editor is editable or not.
  • left.copyLinkEnabled / right.copyLinkEnabled (boolean, optional, default: true). Whether the copy to right/left arrows should appear.


  • diff: the class for a diff line on either editor
  • connector: the SVG connector
  • newCodeConnectorLinkContent: the content of the copy to right link. Defaults to a unicode right arrow ('→')
  • deletedCodeConnectorLinkContent: the content of the copy to left link. Defaults to a unicode right arrow ('←')


There are a few API methods available on your AceDiff instance.

  • aceInstance.getEditors(): this returns an object with left and right properties. Each contains a reference to the Ace editor, in case you need to do anything with them. Ace has a ton of options which I wasn't going to support via the wrapper. This should allow you to do whatever you need
  • aceInstance.setOptions(): this lets you set many of the above options on the fly. Note: certain things used during the construction of the editor, like the classes can't be overridden.
  • aceInstance.getNumDiffs(): returns the number of diffs currently being displayed.
  • aceInstance.diff(): updates the diff. This shouldn't ever be required because AceDiff automatically recognizes the key events like changes to the editor and window resizing. But I've included it because there may always be that fringe case...
  • aceInstance.destroy(): destroys the AceDiff instance. Basically this just destroys both editors and cleans out the gutter.

Browser Support

All modern browsers. Open a ticket if you find otherwise.




npm i ace-diff

DownloadsWeekly Downloads






Unpacked Size

95.5 kB

Total Files


Last publish


  • avatar