How to contribute

Getting help ^

Community discussion, questions, and informal bug reporting is done on the CodeMirror Google group.

The preferred way to report bugs is to use the GitHub issue tracker. Before reporting a bug, read these pointers.

Note: The issue tracker is for bugs, not requests for help. Questions should be asked on the CodeMirror Google group instead.

CodeMirror is maintained by volunteers. They don't owe you anything, so be polite. Reports with an indignant or belligerent tone tend to be moved to the bottom of the pile.
Include information about the browser in which the problem occurred. Even if you tested several browsers, and the problem occurred in all of them, mention this fact in the bug report. Also include browser version numbers and the operating system that you're on.
Mention which release of CodeMirror you're using. Preferably, try also with the current development snapshot, to ensure the problem has not already been fixed.
Mention very precisely what went wrong. "X is broken" is not a good bug report. What did you expect to happen? What happened instead? Describe the exact steps a maintainer has to take to make the problem occur. We can not fix something that we can not observe.
If the problem can not be reproduced in any of the demos included in the CodeMirror distribution, please provide an HTML document that demonstrates the problem. The best way to do this is to go to jsbin.com, enter it there, press save, and include the resulting link in your bug report.

Make sure you have a GitHub Account
Fork CodeMirror (how to fork a repo)
Make your changes
If your changes are easy to test or likely to regress, add tests. Tests for the core go into test/test.js, some modes have their own test suite under mode/XXX/test.js. Feel free to add new test suites to modes that don't have one yet (be sure to link the new tests into test/index.html).
Follow the general code style of the rest of the project (see below). Run bin/lint to verify that the linter is happy.
Make sure all tests pass. Visit test/index.html in your browser to run them.
Submit a pull request (how to create a pull request)

2 spaces per indentation level, no tabs.
Include semicolons after statements.
Note that the linter (bin/lint) which is run after each commit complains about unused variables and functions. Prefix their names with an underscore to muffle it.