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* chore: vendor Mitt into src/common/third-party

As discussed in #6203 we need to vendor our common dependencies in so
that when we ship an ESM build all imports point to file paths and do
not rely on Node resolution (e.g. a browser does not understand `import
mitt from 'mitt'`).
13 contributors

Users who have contributed to this file

@aslushnikov @mathiasbynens @jackfranklin @JoelEinbinder @yanivefraim @vsemozhetbyt @paulirish @mjzffr @MatejMecka @ebidel @MercifulCode @connorjclark
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How to Contribute

First of all, thank you for your interest in Puppeteer! We'd love to accept your patches and contributions!

Contributor License Agreement

Contributions to this project must be accompanied by a Contributor License Agreement. You (or your employer) retain the copyright to your contribution, this simply gives us permission to use and redistribute your contributions as part of the project. Head over to to see your current agreements on file or to sign a new one.

You generally only need to submit a CLA once, so if you've already submitted one (even if it was for a different project), you probably don't need to do it again.

Getting Code

  1. Clone this repository
git clone
cd puppeteer
  1. Install dependencies
npm install
  1. Run Puppeteer tests locally. For more information about tests, read Running & Writing Tests.
npm run unit

Code reviews

All submissions, including submissions by project members, require review. We use GitHub pull requests for this purpose. Consult GitHub Help for more information on using pull requests.

Code Style

  • Coding style is fully defined in .eslintrc and we automatically format our code with Prettier.
  • It's recommended to set-up Prettier into your editor, or you can run npm run eslint-fix to automatically format any files.
  • If you're working in a JS file, code should be annotated with closure annotations.
  • If you're working in a TS file, you should explicitly type all variables and return types. You'll get ESLint warnings if you don't so if you're not sure use them as guidelines, and feel free to ask us for help!

To run ESLint, use:

npm run eslint

You can check your code (both JS & TS) type-checks by running:

npm run tsc

TypeScript guidelines

  • Try to avoid the use of any when possible. Consider unknown as a better alternative. You are able to use any if needbe, but it will generate an ESLint warning.

Project structure and TypeScript compilation

The code in Puppeteer is split primarily into two folders:

  • src contains all source code
  • vendor contains all dependencies that we've vendored into the codebase. See the vendor/ for details.

We structure these using TypeScript's project references, which lets us treat each folder like a standalone TypeScript project.

Shipping CJS and ESM bundles

Currently Puppeteer ships two bundles; a CommonJS version for Node and an ESM bundle for the browser. Therefore we maintain two tsconfig files for each project; tsconfig.esm.json and tsconfig.cjs.json. At build time we compile twice, once outputting to CJS and another time to output to ESM.

We compile into the lib directory which is what we publish on the npm repository and it's structured like so:

- cjs
  - puppeteer <== the output of compiling `src/tsconfig.cjs.json`
  - vendor <== the output of compiling `vendor/tsconfig.cjs.json`
- esm
  - puppeteer <== the output of compiling `src/tsconfig.esm.json`
  - vendor <== the output of compiling `vendor/tsconfig.esm.json`

The main entry point for the Node module Puppeteer is cjs-entry.js. This imports lib/cjs/puppeteer/index.js and exposes it to Node users.

tsconfig for the tests

We also maintain test/tsconfig.test.json. This is only used to compile the unit test *.spec.ts files. When the tests are run, we first compile Puppeteer as normal before running the unit tests against the compiled output. Doing this lets the test run against the compiled code we ship to users so it gives us more confidence in our compiled output being correct.

Root tsconfig.json

The root tsconfig.json exists for the API Extractor; it has to find a tsconfig.json in the project's root directory. It is not used for anything else.

API guidelines

When authoring new API methods, consider the following:

  • Expose as little information as needed. When in doubt, don’t expose new information.
  • Methods are used in favor of getters/setters.
    • The only exception is namespaces, e.g. page.keyboard and page.coverage
  • All string literals must be small case. This includes event names and option values.
  • Avoid adding "sugar" API (API that is trivially implementable in user-space) unless they're very demanded.

Commit Messages

Commit messages should follow the Semantic Commit Messages format:

label(namespace): title


  1. label is one of the following:
    • fix - puppeteer bug fixes.
    • feat - puppeteer features.
    • docs - changes to docs, e.g. docs( .. to change documentation.
    • test - changes to puppeteer tests infrastructure.
    • style - puppeteer code style: spaces/alignment/wrapping etc.
    • chore - build-related work, e.g. doclint changes / travis / appveyor.
  2. namespace is put in parenthesis after label and is optional. Must be lowercase.
  3. title is a brief summary of changes.
  4. description is optional, new-line separated from title and is in present tense.
  5. footer is optional, new-line separated from description and contains "fixes" / "references" attribution to github issues.
  6. footer should also include "BREAKING CHANGE" if current API clients will break due to this change. It should explain what changed and how to get the old behavior.


fix(page): fix method

This patch fixes so that it works with iframes.

Fixes #123, Fixes #234

BREAKING CHANGE: now delivers pizza at home by default.
To deliver to a different location, use "deliver" option:
  `{deliver: 'work'})`.

Writing Documentation

All public API should have a descriptive entry in docs/ There's a documentation linter which makes sure documentation is aligned with the codebase.

To run the documentation linter, use:

npm run doc

Adding New Dependencies

For all dependencies (both installation and development):

  • Do not add a dependency if the desired functionality is easily implementable.
  • If adding a dependency, it should be well-maintained and trustworthy.

A barrier for introducing new installation dependencies is especially high:

  • Do not add installation dependency unless it's critical to project success.

There are additional considerations for dependencies that are environment agonistic. See the vendor/ for details.

Running & Writing Tests

  • Every feature should be accompanied by a test.
  • Every public api event/method should be accompanied by a test.
  • Tests should not depend on external services.
  • Tests should work on all three platforms: Mac, Linux and Win. This is especially important for screenshot tests.

Puppeteer tests are located in the test directory (test and are written using Mocha. See test/ for more details.

Despite being named 'unit', these are integration tests, making sure public API methods and events work as expected.

  • To run all tests:
npm run unit
  • To run a specific test, substitute the it with it.only:
  it.only('should work', async function({server, page}) {
    const response = await page.goto(server.EMPTY_PAGE);
  • To disable a specific test, substitute the it with xit (mnemonic rule: 'cross it'):
  // Using "xit" to skip specific test
  xit('should work', async function({server, page}) {
    const response = await page.goto(server.EMPTY_PAGE);
  • To run tests in non-headless mode:
HEADLESS=false npm run unit
  • To run Firefox tests, firstly ensure you have Firefox installed locally (you only need to do this once, not on every test run) and then you can run the tests:
PUPPETEER_PRODUCT=firefox node install.js
PUPPETEER_PRODUCT=firefox npm run unit
  • To run tests with custom browser executable:
BINARY=<path-to-executable> npm run unit

Public API Coverage

Every public API method or event should be called at least once in tests. To ensure this, there's a coverage command which tracks calls to public API and reports back if some methods/events were not called.

Run coverage:

npm run coverage

Debugging Puppeteer

See Debugging Tips in the readme.

For Project Maintainers

Releasing to npm

Releasing to npm consists of the following phases:

  1. Source Code: mark a release.
    1. Bump package.json version following the SEMVER rules.
    2. Run npm run doc to update the docs accordingly.
    3. Update the “Releases per Chromium Version” list in docs/ to include the new version. Note: only do this when the Chrome revision is different from the previous release.
    4. Send a PR titled 'chore: mark version vXXX.YYY.ZZZ' (example).
    5. Make sure the PR passes all checks.
      • WHY: there are linters in place that help to avoid unnecessary errors, e.g. like this
    6. Merge the PR.
    7. Once merged, publish the release notes using GitHub's “draft new release tag” option.
      • NOTE: tag names are prefixed with 'v', e.g. for version 1.4.0 the tag is v1.4.0.
      • For the “raw notes” section, use git log --pretty="%h - %s" v2.0.0..HEAD.
  2. Publish puppeteer to npm.
    1. On your local machine, pull from upstream and make sure the last commit is the one just merged.
    2. Run git status and make sure there are no untracked files.
      • WHY: this is to avoid adding unnecessary files to the npm package.
    3. Run npx pkgfiles to make sure you don't publish anything unnecessary.
    4. Run npm publish. This publishes the puppeteer package.
  3. Publish puppeteer-core to npm.
    1. Run ./utils/prepare_puppeteer_core.js. The script changes the name inside package.json to puppeteer-core.
    2. Run npm publish. This publishes the puppeteer-core package.
    3. Run git reset --hard to reset the changes to package.json.
  4. Source Code: mark post-release.
    1. Bump package.json version to -post version, run npm run doc to update the “released APIs” section at the top of docs/ accordingly, and send a PR titled 'chore: bump version to vXXX.YYY.ZZZ-post' (example)
      • NOTE: no other commits should be landed in-between release commit and bump commit.

Updating npm dist tags

For both puppeteer and puppeteer-core we maintain chrome-* npm dist tags, e.g. chrome-75 and so on. These tags match the Puppeteer version that corresponds to the chrome-* release.

These tags are updated on every Puppeteer release.

Managing tags 101:

# List tags
$ npm dist-tag ls puppeteer
# Add tags
$ npm dist-tag add puppeteer@3.0.0 chrome-81
$ npm dist-tag add puppeteer-core@3.0.0 chrome-81
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