PostHog Browser JS Library

For information on using this library in your app, see PostHog Docs.
This README is intended for developing the library itself.

Testing

Unit tests: run pnpm test. Cypress: run pnpm start to have a test server running and separately pnpm cypress to launch Cypress test engine.

Running TestCafe E2E tests with BrowserStack

Testing on IE11 requires a bit more setup. TestCafe tests will use the playground application to test the locally built array.full.js bundle. It will also verify that the events emitted during the testing of playground are loaded into the PostHog app. By default it uses https://us.i.posthog.com and the project with ID 11213. See the testcafe tests to see how to override these if needed. For PostHog internal users ask @benjackwhite or @hazzadous to invite you to the Project. You'll need to set POSTHOG_API_KEY to your personal API key, and POSTHOG_PROJECT_KEY to the key for the project you are using.

You'll also need to sign up to BrowserStack. Note that if you are using CodeSpaces, these variables will already be available in your shell env variables.

After all this, you'll be able to run through the below steps:

1. Optional: rebuild array.js on changes: nodemon -w src/ --exec bash -c "pnpm build-rollup".
2. Export browserstack credentials: export BROWSERSTACK_USERNAME=xxx BROWSERSTACK_ACCESS_KEY=xxx.
3. Run tests: npx testcafe "browserstack:ie" testcafe/e2e.spec.js.

Running local create react app example

You can use the create react app setup in playground/nextjs to test posthog-js as an npm module in a Nextjs application.

1. Run posthog locally on port 8000 (DEBUG=1 TEST=1 ./bin/start).
2. Run python manage.py setup_dev --no-data on posthog repo, which sets up a demo account.
3. Copy Project API key found in http://localhost:8000/project/settings and save it for the last step.
4. Run cd playground/nextjs.
5. Run pnpm i to install dependencies.
6. Run pnpm run build-posthog-js to build posthog-js locally.
7. Run NEXT_PUBLIC_POSTHOG_KEY='<your-local-api-key>' NEXT_PUBLIC_POSTHOG_HOST='http://localhost:8000' pnpm dev to start the application.

Tiers of testing

1. Unit tests - this verifies the behavior of the library in bite-sized chunks. Keep this coverage close to 100%, test corner cases and internal behavior here
2. Cypress tests - integrates with a real chrome browser and is capable of testing timing, browser requests, etc. Useful for testing high-level library behavior, ordering and verifying requests. We shouldn't aim for 100% coverage here as it's impossible to test all possible combinations.
3. TestCafe E2E tests - integrates with a real posthog instance sends data to it. Hardest to write and maintain - keep these very high level

Developing together with another project

Install pnpm to link a local version of posthog-js in another JS project: npm install -g pnpm

Run this to link the local version

We have 2 options for linking this project to your local version: via pnpm link or via local paths

local paths (preferred)

• from whichever repo needs to require posthog-js, go to the package.json of that file, and replace the posthog-js dependency version number with file:<relative_or_absolute_path_to_local_module>
• e.g. from the package.json within posthog, replace "posthog-js": "1.131.4" with "posthog-js": "file:../posthog-js"
• run pnpm install from the root of the project in which you just created a local path

Then, once this link has been created, any time you need to make a change to posthog-js, you can run pnpm build from the posthog-js root and the changes will appear in the other repo.

pnpm link

• In the posthog-js directory: pnpm link --global
• (for posthog this means: pnpm link --global posthog-js && pnpm i && pnpm copy-scripts)
• You can then remove the link by, e.g., running pnpm link --global posthog-js from within posthog

Releasing a new version

Just put a bump patch/minor/major label on your PR! Once the PR is merged, a new version with the appropriate version bump will be released, and the dependency will be updated in posthog/PostHog – automatically.

If you forget to add the label, don't try to update the version locally as you won't be able to push that commit to the main branch. Instead, just make a new PR.

Prereleases

To release an alpha or beta version, you'll need to use the CLI locally:

CLI

Only one person is set as a collaborator on NPM, so they're the only person that can manually publish alphas

1. Make sure you're a collaborator on posthog-js in npm (check here).

2. Make sure you're logged into the npm CLI (npm login).

3. Check out your work-in-progress branch (do not release an alpha/beta from main).

4. Run the following commands, using the same bump level (major/minor/patch) as your PR:

   npm version [premajor | preminor | prepatch] --preid=beta
   npm publish --tag beta
   git push --tags

5. Enjoy the new prerelease version. You can now use it locally, in a dummy app, or in the main repo.

Automagically

Use the "release alpha" label on your PR to have an alpha version published automatically. This automation currently doesn't check whether an alpha exists for the version it will try to publish. If you need to publish two alphas from one PR you'll need to fix that

Remember that these versions are public and folk might use them, so make sure they're not too alpha 🙈