Prepack

Prepack is a partial evaluator for JavaScript. Prepack rewrites a JavaScript bundle, resulting in JavaScript code that executes more efficiently. For initialization-heavy code, Prepack works best in an environment where JavaScript parsing is effectively cached.

See the official prepack.io website for an introduction and an interactive REPL playground.

How to use Prepack

Install the CLI via npm,

$ npm install -g prepack

Or if you prefer yarn,

$ yarn global add prepack

You may need to prepend (pun intended!) the command with sudo in some cases.

Let the party begin

To compile a file and print the output to the console:

$ prepack script.js

If you want to compile a file and output to another file:

$ prepack script.js --out script-processed.js

Detailed instructions and the API can be found at Prepack CLI: Getting Started

Status

• test262 status on master branch
• code coverage report for serialization tests
• To see the status for a pull request, look for the message All checks have passed or All checks have failed. Click on Show all checks, Details, Artifacts, and then test262-status.txt or coverage-report-sourcemapped/index.html.

How to get the code

0. Clone repository and make it your current directory.
1. git submodule init
2. git submodule update --init
3. Get npm and node, then do npm install

How to build, lint, type check

0. Get the code
1. npm run build
   You can later run npm run watch in the background to just compile changed files on the fly.
2. npm run lint
3. npm run flow

How to run tests

0. Get the code
1. Make sure the code is built, either by running npm run build or npm run watch
2. npm test

You can run individual test suites as follows:

• npm run test-serializer
  This tests the interpreter and serializer. All tests should pass.
• npm run test-test262
  This tests conformance against the test262 suite. Not all will pass, increasing conformance is work in progress.

How to run the interpreter

0. Get the code
1. Make sure the code is built, either by running npm run build or npm run watch
2. npm run repl
   This starts an interactive interpreter session.

How to run Prepack

0. Get the code

1. Make sure the code is built, either by running npm run build or npm run watch.

2. Have a JavaScript file handy that you want to prepack, for example:
   echo "function hello() { return 'hello'; } function world() { return 'world'; } s = hello() + ' ' + world();" >/tmp/sample.js

3. cat /tmp/sample.js | npm run prepack
   Try --help for more options.

How to validate changes

Instead of building, linting, type checking, testing separately, the following does everything together:
npm run validate

How to edit the website

The content for prepack.io resides in the gh-pages branch of this repository. To make changes, submit a pull request, just like for any code changes. In order to run the website locally at localhost:8000, run python -m SimpleHTTPServer (Python 2) or python -m http.server (Python 3) from the cloned gh-pages branch.

At this time, a particular bundled version of Prepack is checked in to the gh-pages branch at js/prepack.min.js. To update the bundle, run npm run build-bundle from the master branch, and copy the resulting prepack.min.js file into the gh-pages branch into the js directory, and submit a pull request for that change.

How to contribute

For more information about contributing pull requests and issues, see our Contribution Guidelines.

License

Prepack is BSD-licensed. We also provide an additional patent grant.