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Contributing to OpenLayers

Thanks for your interest in contributing to OpenLayers.

Asking Questions

Please ask questions about using the library on Stack Overflow using the tag 'openlayers'.

When you want to get involved and discuss new features or changes, please use the mailing list.

Submitting Bug Reports

Please use the GitHub issue tracker. Before creating a new issue, do a quick search to see if the problem has been reported already.

Getting Familiar with the Code in the Repository

Look for readme.md files! Several directories contain a readme.md file that explains the contents of the directory and how to work with them.

Contributing Code

See DEVELOPING.md to learn how to get started developing.

Our preferred means of receiving contributions is through pull requests. Make sure that your pull request follows our pull request guidelines below before submitting it.

This page describes what you need to know to contribute code to OpenLayers as a developer.

Contributor License Agreement

Before accepting a contribution, we ask that you provide us a Contributor License Agreement. If you are making your contribution as part of work for your employer, please follow the guidelines on submitting a Corporate Contributor License Agreement. If you are making your contribution as an individual, you can submit a digital Individual Contributor License Agreement.

Pull request guidelines

Before working on a pull request, create an issue explaining what you want to contribute. This ensures that your pull request won't go unnoticed, and that you are not contributing something that is not suitable for the project. Once a core developer has set the pull request accepted label on the issue, you can submit a pull request. The pull request description should reference the original issue.

Your pull request must:

  • Follow OpenLayers's coding style.

  • Pass the integration tests run automatically by the Travis Continuous Integration system.

  • Address a single issue or add a single item of functionality.

  • Contain a clean history of small, incremental, logically separate commits, with no merge commits.

  • Use clear commit messages.

  • Be possible to merge automatically.

The check build target

It is strongly recommended that you run

$ make check

before every commit. This will catch many problems quickly, and it is much faster than waiting for the Travis CI integration tests to run.

The check build target runs a number of quick tests on your code. These include:

  • Lint
  • Compile
  • Tests

Follow OpenLayers's coding style

OpenLayers follows Google's JavaScript Style Guide. This is checked using ESLint, you can run the linter locally on your machine before committing using the lint target:

$ make lint

In addition to fixing problems identified by the linter, please also follow the style of the existing OpenLayers code, which includes:

  • Always wrap the body of for, if, and while statements in braces.

  • Class methods should be in alphabetical order.

  • var declarations should not span multiple lines. If you cannot fit all the declarations in a single line, then start a new var declaration on a new line. Within a single line, variables should be declared in alphabetical order.

  • Do not use assignments inside expressions.

  • Use uppercase for @const variables.

Configure your editor

If possible, configure your editor to follow the coding conventions of the library. A .editorconfig file is included at the root of the repository that can be used to configure whitespace and charset handling in your editor. See that file for a description of the conventions. The EditorConfig site links to plugins for various editors.

Pass the integration tests run automatically by the Travis CI system

The integration tests contain a number of automated checks to ensure that the code follows the OpenLayers style and does not break tests or examples. You can run the integration tests locally using the ci target:

$ make ci

Address a single issue or add a single item of functionality

Please submit separate pull requests for separate issues. This allows each to be reviewed on its own merits.

Contain a clean history of small, incremental, logically separate commits, with no merge commits

The commit history explains to the reviewer the series of modifications to the code that you have made and breaks the overall contribution into a series of easily-understandable chunks. Any individual commit should not add more than one new class or one new function. Do not submit commits that change thousands of lines or that contain more than one distinct logical change. Trivial commits, e.g. to fix lint errors, should be merged into the commit that introduced the error. See the Atomic Commit Convention on Wikipedia for more detail.

git apply --patch and git rebase can help you create a clean commit history. Reviewboard.org and Pro GIT have explain how to use them.

Use clear commit messages

Commit messages should be short, begin with a verb in the imperative, and contain no trailing punctuation. We follow http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html for the formatting of commit messages.

Git commit message should look like:

Header line: explaining the commit in one line

Body of commit message is a few lines of text, explaining things
in more detail, possibly giving some background about the issue
being fixed, etc etc.

The body of the commit message can be several paragraphs, and
please do proper word-wrap and keep columns shorter than about
74 characters or so. That way "git log" will show things
nicely even when it's indented.

Further paragraphs come after blank lines.

Please keep the header line short, no more than 50 characters.

Be possible to merge automatically

Occasionally other changes to master might mean that your pull request cannot be merged automatically. In this case you may need to rebase your branch on a more recent master, resolve any conflicts, and git push --force to update your branch so that it can be merged automatically.

Building on Windows

Most developers build on Linux. Building on Windows is possible under Cygwin. When installing Cygwin from https://www.cygwin.com/, include the developer tools to get GNU make.

First (before npm install), to avoid file permission problems between Windows and Cygwin, edit Cygwin's /etc/fstab file to disable ACLs like this none /cygdrive cygdrive binary,noacl,posix=0,user 0 0

Python is normally installed with Cygwin so need not be installed separately. By default Cygwin will use its own version of Python rather than Window's, so the Python modules should be installed for Cygwin's Python.

The build targets check-deps, serve, lint, build, test, check and host-examples described above should all work. host-examples takes quite a while to run. If a target does not run properly first time, try it again.

Currently, Firefox fails to run http://localhost:3000/build/examples from make serve, but Chrome and Internet Explorer will.

Microsoft Visual Studio's javascript debugger may be used to debug the build/hosted/your-branch/examples. It will be convenient to set build/hosted/your-branch/examples/index.html as the startup page.

Your OpenLayers source tree need not be under the Cygwin root. if you checkout to c:/openlayers then you can build under Cygwin at /cygdrive/c/openlayers . However, keep the path to the OpenLayers files short otherwise you may see ENAMETOOLONG errors.