Contribute to this guideReport an issue

guideDevelopment environment

The CKEditor 5 codebase is divided into multiple npm packages, each developed in a separate Git repository. The main package is ckeditor5 which installs all project dependencies and various development-related resources such as:

  • the testing environment setup,
  • configuration for mgit (a multi-repo management tool) and Lerna.js (a multi-package management tool),
  • translation management tools,
  • documentation generator,
  • and release tools.

You can find all the official packages listed in CKEditor 5 development repository’s README.

# Requirements

In order to start developing CKEditor 5 you will require:

  • Node.js 6.9.0+
  • npm 4+ (note: some npm 5+ versions were known to cause problems, especially with deduplicating packages; upgrade npm when in doubt)
  • Git

# Setting up the CKEditor development environment

First, you need to install a couple of tools which you will be using later:

It is best to install them globally in your system for an easier use later on:

npm install -g lerna@2 mgit2

Note: You may need to use sudo on Linux and macOS.

Then clone the main repository:

git clone
cd ckeditor5

And install all CKEditor 5 packages from the npm registry.

Note: If you plan to use the developement version of CKEditor 5 packages (see the next section), you can skip this step to save time.

npm install

This may take a while

# Switching to development version of packages

The steps above should install all the packages from npm, which means that you will have the latest releases of all of them. They are available in node_modules/@ckeditor/ (we are using scoped packages, hence the unusual directory).

In order to work with development versions of all the official packages, it is recommended to use mgit and Lerna. The former will clone all package repositories and the latter will be able to symlink them, so they create a correct directory structure, understandable by Node.js-compliant tools (like webpack or Browserify).

First, clone all the repositories:

mgit bootstrap

Expected result:

(master 340feac) p@m /workspace/ckeditor5> tree -L 1 packages/
├── ckeditor5-autoformat
├── ckeditor5-basic-styles
├── ckeditor5-clipboard
└── ckeditor5-utils

Finally, link them:

lerna bootstrap

Running Lerna may take a while because it installs all package dependencies. It will also warn you about circular dependencies between packages which you can ignore.

Now, all CKEditor packages (except the development tools) should be cross-symlinked:

(master 340feac) p@m /workspace/ckeditor5> ls -la node_modules/\@ckeditor/ckeditor5-utils/node_modules/\@ckeditor/
total 16
drwxr-xr-x    5 p  staff   170 31 Jan 10:37 .
drwxr-xr-x  292 p  staff  9928 20 Jan 00:20 ..
lrwxr-xr-x    1 p  staff    23 31 Jan 10:37 ckeditor5-core -> ../../../ckeditor5-core
drwxr-xr-x    7 p  staff   238 20 Jan 00:20 ckeditor5-dev-docs
lrwxr-xr-x    1 p  staff    25 31 Jan 10:37 ckeditor5-engine -> ../../../ckeditor5-engine

If everything worked correctly, you should be able to run some tests:

npm run test -- --files=core

# Fetching changes

Whenever you want to update all packages to their latest versions call:

# To update the ckeditor5 repository itself:
git pull

# To update pull changes to all the packages:
mgit update

From time to time, if the list of dependencies in any of the packages changed, you will need to call Lerna again to symlink them:

lerna bootstrap

You can also speed it up if you know which package has changed:

lerna bootstrap --scope=@ckeditor/ckeditor5-core

# Using mgit for custom packages

If you are developing custom packages or forked any of the official packages and want mgit to work with it, change the dependencies configuration in mgit.json. Note that mgit is able to clone the package from any Git URL. Refer to its documentation for more details.

# Troubleshooting problems with Lerna

Lerna does pretty complicated things on an already complicated npm ecosystem. If you happen to run into some issues when calling lerna bootstrap, here are some tips:

  • Look for npm-debug.log files in the main package and subpackages. They may point to an obvious issue like a typo in some package.json.
  • Sometimes repeating lerna bootstrap may help.
  • If nothing else works, do lerna clean && lerna bootstrap.

# Final word about mgit and Lerna

Besides the already mentioned features, mgit allows you to execute shell commands on all packages (e.g. check their status). It has been developed by the CKSource team and we are relying on it heavily, hence you can expect more features and improvements to come. However, it is not a CKEditor-specific tool and should be suitable for any multi-repository project (though it best fits JavaScript projects).

Lerna is a tool used by many well-known projects such as Babel.js. It has an amazing community and, relying on it ourselves, we hope that it will become a standard for managing multi-package projects.

# Running tests

In order to run tests you need to use the test and manual tasks.

npm test -- --watch --coverage --source-map --files=engine

or, shorter:

npm test -- -wcs --files=engine

This command will run the ckeditor5-engine package’s tests.

Note: It is not possible to run tests of all packages with code coverage at once because the size of the project (the number of test files and source modules) exceeds webpack’s capabilities (it runs out of memory).

To create a server for manual tests use the manual task:

npm run manual

It accepts the --source-map (-s) option. Note that it watches for changes in the JavaScript files only (see the bug).

You can read more about the Testing environment.

# Generating documentation

To build the documentation you need to run the docs task:

npm run docs

The documentation will be available in build/docs/.

This task accepts the following arguments:

  • --skip-api – Skips building the API documentation (which takes the majority of the total time).
  • --skip-snippets – Skips building live snippets.
  • --skip-validation – Skips the final link validation.
  • --watch – Runs the documentation generator in a watch mode. It covers guides (it does not cover API docs).
  • --production – Minifies the assets and performs other actions which are unnecessary during CKEditor 5 development.
  • --verbose – Prints out more information.

Note: These arguments must be passed after additional --:

npm run docs -- --skip-api

# Bisecting through a multi-repository

CKEditor 5 is a multi-repository project. It means that git bisect (which is super handy when tracking which commit introduced a bug) will not work out of the box.

Fortunately, every commit made to any of the master branches of all CKEditor 5 subpackages will update this subpackage’s hash in mgit.json in the master-revisions branch.

Thanks to that, master-revisions contains an ordered history of all changes which makes it possible to go back to any point in history:

# Make sure to update this branch.
git co master-revisions
git pull

# Check out 30 commits back.
git co master-revisions~30

# Check out subpackages to correct hashes.
mgit co

Once you found the point in history which interests you, you can go straight to a commit in a subpackage and PR. For example:

(detached:bisect/bad~1 cb2feef ?1) p@m /workspace/ckeditor5> git bisect good
ab5b5494e5eba7beb4a3bac4ea9c5e6d59a610f5 is the first bad commit
commit ab5b5494e5eba7beb4a3bac4ea9c5e6d59a610f5
Author: Travis CI User <>
Date:   Tue Apr 11 16:45:47 2017 +0000


:100644 100644 611526d5c4beae7046b0a08ec36843cbbe642175 35acdd8ba971d8127dd462a9c8c1162a54f00d58 M    mgit.json

Leads to ckeditor/ckeditor5-image@02869eb which leads to ckeditor/ckeditor5-image#95.

# Additional information for contributors

# SVG icons

By default, CKEditor 5 supports SVG icons found in the ckeditor5-*/theme/icons folders. Unfortunately, most of the SVG editing software produces the output with comments, obsolete tags, and complex paths, which bloats the DOM and makes the builds heavy for no good reason.

To remove the excess data and prevent certain issues, all new icons should be optimized before joining the code base. The right utility to do this is Node–based SVGO and the usage is as simple as:

npm install -g svgo
cd ckeditor5-package-name/theme/icons
svgo --enable removeTitle -i .

SVGO reduces the icon size up to 70%, depending on the software used to create it and the general complexity of the image.