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.

Status

We, the Prepack team at Facebook, have temporarily set down work on Prepack including the React compiler project. You won't see many Prepack PRs while we are currently prioritizing some other projects.

How to use Prepack

Install the CLI via npm,

$ npm install -g prepack

Or if you prefer yarn, make sure you get yarn first,

$ npm install -g yarn

and then install the Prepack CLI via 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

Plugins to other tools

The following are a few plugins to other tools. They have been created and are maintained separately from Prepack itself. If you run into any issues with those plugins, please ask the plugin maintainers for support.

Test Results and Code Coverage

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

Clone repository and make it your current directory.
git submodule init
git submodule update --init
Get yarn and node, then do yarn

Note: For development work you really need yarn, as many scripts require it.

How to build, lint, type check

Get the code
yarn build
You can later run yarn watch in the background to just compile changed files on the fly.
yarn lint
yarn flow

How to run tests

Get the code
Make sure the code is built, either by running yarn build or yarn watch
yarn test

You can run individual test suites as follows:

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

How to run the interpreter

Get the code
Make sure the code is built, either by running yarn build or yarn watch
yarn repl
This starts an interactive interpreter session.

How to run Prepack

Get the code
Make sure the code is built, either by running yarn build or yarn watch.
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
cat /tmp/sample.js | yarn prepack-cli
Try --help for more options.

How to validate changes

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

How to edit the website

The content for prepack.io resides in the website directory 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:

Build prepack into the website: yarn build && mv prepack.min.js website/js
Run python -m SimpleHTTPServer (Python 2) or python -m http.server (Python 3) from the website/ directory