We use the Yarn package manager to manage dependencies and run the scripts defined in
package.json. It is similar to npm, but contains fewer bugs for our purposes. If you make changes which add or remove dependencies, make sure you commit the
yarn.lock file which it generates.
You can probably get away with using npm for most contributions, as long as you are not adding or removing dependencies. You'll have to mentally translate the Yarn commands we give into npm ones, however.
jsdom is a blend of old and new code. Some of its older and less-touched corners may look different from newer work. Here we'll describe the modern setup, but you might encounter parts of the codebase that don't fit this model, or that seem unnecessarily baroque (like the directory structure in
In general, a web platform class (like
CSSStyleSheet) is specified using a language called Web IDL. Web IDL abstracts away a lot of the boilerplate involved in creating such classes, like type conversions, argument validation, and attribute/property reflection.
As such, most web platform classes present in jsdom are implemented in two parts:
An IDL file, such as
Attr.webidl, drawn more or less straight from the spec
An implementation file, such as
Attr-impl.js, containing the relevant implementation logic
Our build step (
yarn prepare) then generates a public API file (e.g.
Attr.js) which takes care of all the Web IDL-derived boilerplate, delegating to the implementation file for the important stuff. We then wire it together with a line in
lib/jsdom/living/interfaces.js that exposes the generated class on all jsdom windows.
When contributing, the first question you should ask is:
Can I exhibit how the browsers differ from what jsdom is doing?
If you can, then you've almost certainly found a bug in or missing feature of jsdom, and we'd love to have your contribution. In that case, move on to:
What spec covers this potential contribution?
Almost all of our relevant functionality is covered in either the DOM Living Standard or the HTML Living Standard. There are various obsolete W3C specs ("DOM Level 2" etc.) that were never really implemented in browsers, and there is also the "DOM Level 4" W3C fork of the WHATWG DOM Living Standard. But we try to stick to the two main WHATWG specs for jsdom these days.
Other specs might pop up from time to time, especially in regard to CSS stuff. In general Mozilla's Servo project provides good guidance on relevant places to look. platform.html5.org is also pretty comprehensive.
Running the tests
First you'll want to run
yarn to install all dependencies. Then, configure your system to run the web platform tests as described in their README.
To run all the tests:
In the following sections, we'll give commands for running a subset of the tests. If you do that, instead of running the whole suite with
yarn test, then please run this command first:
Before running test subsets:
Web platform feature tests
All tests for web platform features (as opposed to features of jsdom itself, such as the
JSDOM() constructor) should be in web-platform-tests format. We have some infrastructure for running these directly against jsdom documents. So ideally, when contributing a bugfix or new feature, you can browse the web-platform-tests repository and find the test covering your work, and then just enable it in the
to-run.yaml file. These tests are HTML files which use a special library called testharness.js to report their results.
However, the web-platform-tests project is not fully comprehensive. If you need to write your own test for a web platform feature, place it in our to-upstream directory. (It's so named because, over time, we hope to upstream these tests back to the web-platform-tests repository, so all browsers can benefit from them.) Note that you may need to create new directory structure, paralleling that of the main web-platform-tests repository.
To run all web-platform-tests:
To run the to-upstream web-platform-tests:
To run specific web-platform-tests already enabled via
yarn test-wpt --fgrep dom/events
To run specific to-upstream web-platform-tests:
yarn test-tuwpt --fgrep domparsing
Also, to update web platform tests to their latest revision from the source repository:
yarn update-wpt. (This can take a long time, like 10 minutes.)
jsdom API tests
If you are testing something that can only be accomplished through the jsdom API, and not inside a normal web browser, you'll want to write a different kind of test. Such tests are written using Mocha and Chai.
To write such a test that, simply add a file in
test/api/, following the surrounding conventions. Then, add it to the manifest at
To run all API tests:
To run a specific API test:
yarn test-mocha test/api/from-file.js
Although ideally you should not need to worry about this, there are some tests that are for legacy reasons not in the right format; they use Mocha, but really should be web platform tests. We're keeping them around for coverage until we can convert them. If you run
yarn test, you will get the full test suite, including such old tests.
Testing against the browser
The mocha test cases are executed in Chrome using karma. Currently, web platform tests are not executed in the browser yet.
To run all browser tests:
To run the karma tests in an iframe:
To run the karma tests in a web worker:
This project cares about performance. There are a number of benchmarks that you can run. If you suspect your contribution has an impact on the performance of existing functionality, make sure you run the benchmarks before and after your change so that you can compare.
You can also run the benchmarks using the native DOM implementation of Chrome. A comparison with jsdom will automatically be made for you. If your new feature is much slower than the alternative DOM implementation, there might be an unexpected bottleneck somewhere in your change.
To run benchmarks in Node.js:
To run benchmarks in the browser:
yarn benchmark-browser, then open
benchmark/browser-runner.html in Chrome (or Chromium) and use the developer console to execute the
If you've read this far, you should know everything there is to know about contributing to jsdom. We have an active and full issue tracker that we'd love you to help with. Go find something broken or missing, and fix it!