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A JavaScript bundle optimizer.

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Prepack Circle CI

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 website for an introduction and an interactive REPL playground.


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

(pun intended!) the command with
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

How to get the code

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

Note: For development work you really need

, as many scripts require it.

How to build, lint, type check

  1. Get the code
  2. yarn build

    You can later run
    yarn watch
    in the background to just compile changed files on the fly.
  3. yarn lint
  4. yarn flow

How to run tests

  1. Get the code
  2. Make sure the code is built, either by running
    yarn build
    yarn watch
  3. 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

  1. Get the code
  2. Make sure the code is built, either by running
    yarn build
    yarn watch
  3. yarn repl

    This starts an interactive interpreter session.

How to run Prepack

  1. Get the code
  2. Make sure the code is built, either by running
    yarn build
    yarn watch
  3. 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
  4. cat /tmp/sample.js | yarn prepack-cli

    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 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: 1. Build prepack into the website:

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

How to contribute

To read more about the project, check out this suggested reading wiki

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


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

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