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1.128.2 • Public • Published

PostHog Browser JS Library

npm package MIT License

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


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 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 setup_dev --no-data on posthog repo, which sets up a demo account.
  3. Copy posthog token found in http://localhost:8000/project/settings and then
  4. cd playground/nextjsand run NEXT_PUBLIC_POSTHOG_KEY='<your-local-api-key>' pnpm dev

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

Run this to link the local version

  • In the posthog-js directory: yalc publish
  • In the other directory: yalc add posthog-js, then install dependencies
    (for posthog this means: yalc add posthog-js && pnpm i && pnpm copy-scripts)

Run this to update the linked local version

  • In the other directory: yalc update, then install dependencies
    (for posthog this means: yalc update && pnpm i && pnpm copy-scripts)

Run this to unlink the local version

  • In the other directory: yalc remove posthog-js, then install dependencies
    (for posthog this means: yalc remove posthog-js && pnpm i && pnpm copy-scripts)

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.


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

  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.




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