7.9 KB

Contributing to KaTeX

We welcome pull requests to KaTeX. If you'd like to add a new symbol, or try to tackle adding a larger feature, keep reading. If you have any questions, or want help solving a problem, feel free to stop by our gitter channel.

Helpful contributions

If you'd like to contribute, try contributing new symbols or functions that KaTeX doesn't currently support. The documentation has pages listing supported functions and functions that KaTeX supports and some that it doesn't support. You can check them to see if we don't support a function you like, or try your function in the interactive demo at The wiki has a page that describes how to examine TeX commands and where to find rules which can be quite useful when adding new commands.

Single symbols

There are many individual symbols that KaTeX doesn't yet support. Read through the symbols.js file for more information on how to add a symbol.

To figure out the unicode symbol for the symbol you are trying to add, try using the symbol in MathJax to see what unicode symbol it outputs. An interactive MathJax shell can be found here.

To figure out what group your symbol falls into, look through the symbols list to find other symbols of a similar kind. (e.g. if you were adding \neq, look for =). If you cannot find anything similar, or are unsure, you can try using your symbol in TeX surrounded by other different kinds of symbols, and seeing whether your spacing matches the spacing that TeX produces.

Once your symbol works, check the JavaScript console to make sure you don't get a message like "Can't find character metrics for _" when you render your symbol. If you do, check out

Adding new functions

New functions should be added in src/functions using defineFunction from defineFunction.js. Read the comments in this file to get started. Look at phantom.js and delimsizing.js as examples of how to use defineFunction. Notice how delimsizing.js groups several related functions together in a single call to defineFunction.

The new method of defining functions combines methods that were previously spread out over three different files functions.js, buildHTML.js, buildMathML.js into a single file. The goal is to have all functions use this new system.


Local testing can be done by running the webpack-dev-server using configuration Run yarn to install dependencies, and then yarn start to start the server.

This will host an interactive editor at http://localhost:7936/ to play around with and test changes.

webpack-dev-server 2.8.0 introduced a change which included ES6 keywords const and let within the scripts being served to the browser, and therefore doesn't support IE 9 and 10. If you want to test in IE 9 and 10, install version 2.7.1 by running yarn add webpack-dev-server@2.7.1.

Jest tests

The JavaScript parser and some of the HTML and MathML tree builders are tested with Jest. These tests can be run using node with yarn test:jest. If you need to debug the tests see

The interactive editor can also be used for debugging tests in the browser by copy/pasting the test case to be debugged into the editor. The permalink option can come in really useful when doing repeated runs of the same test case.

The Jest tests should be run after every change, even the addition of small symbols. However, CircleCI will run these tests when you submit a pull request, in case you forget.

If you make any changes to Parser.js, add Jest tests to ensure they work.

Some tests verify the structure of the output tree using snapshot testing. Those snapshots can be updated by running yarn test:jest:update.

Also, test code coverage can be collected by yarn test:jest:coverage. You can view the report in coverage/lcov-report/index.html.

Screenshot tests

To ensure the final output looks good, we screenshot different expressions. These tests can be run by using the screenshotter docker.

The screenshot tests should be run if you add anything more significant than individual symbols. These tests are not automatically run, so please remember! If the new images are different (meaning they are not byte-by-byte the same as the old ones), inspect them visually. If there are no visible changes, that is okay. If things change in a way consistent with your additions, explain what changed and why. Otherwise, figure out what is causing the changes and fix it!

If you add a feature that is dependent on the final output looking the way you created it, add a screenshot test. See ss_data.yaml.

You can use our texcmp tool to compare the outputs of a screenshot test as generated by KaTeX and LaTeX. It's often useful to attach the resulting "visual diff" to your pull request with a new feature.

Testing in other browsers

KaTeX supports all major browsers, including IE 9 and newer. Unfortunately, it is hard to test new changes in many browsers. If you can, please test your changes in as many browsers as possible. In particular, if you make CSS changes, try to test in IE 9, using VMs.


KaTeX is built using webpack with configuration webpack.config.js. Run yarn build to build the project.

Style guide


  • 4 spaces for indentation
  • 80 character line length
  • commas last
  • declare variables in the outermost scope that they are used
  • camelCase for variables in JavaScript
  • snake_case for variables in Python

In general, try to make your code blend in with the surrounding code.

The code can be linted by running yarn test:lint, which lints JavaScript files using ESLint and stylesheets using stylelint. They must pass to commit the changes.

Some files have flowtype annotations and can be checked for type errors using Flow by running yarn test:flow. See Flow for more details.

Pull Requests

  • link back to the original issue(s) whenever possible
  • new commands should be added to docs/ and docs/
  • commits should be squashed before merging
  • large pull requests should be broken into separate pull requests (or multiple logically cohesive commits), if possible

Working with submodules

The fonts for KaTeX live in a submodule stored in submodules/katex-fonts. When you first clone the KaTeX repository, use git submodule update --init --recursive to download the corresponding fonts repository. After running yarn, you should have Git hooks that will automatically run this command after switching to branches where submodules/katex-fonts point to different commits.

When submitting pull requests that update katex-fonts, you'll need to submit two pull requests: one for KaTeX/katex-fonts and one for KaTeX/KaTeX.

For more info about how to use git submodules, see


In order to contribute to KaTeX, you must first sign the CLA, found at


KaTeX is licenced under the MIT License.