metro/packages/metro-bundler

React Native Packager

React Native Packager is a project similar in scope to browserify or webpack, it provides a CommonJS-like module system, JavaScript compilation (ES6, Flow, JSX), bundling, and asset loading.

The main difference is the Packager's focus on compilation and bundling speed. We aim for a sub-second edit-reload cycles. Additionally, we don't want users -- with large code bases -- to wait more than a few seconds after starting the packager.

The main deviation from the node module system is the support for our proprietary module format known as @providesModule. However, we discourage people from using this module format because going forward we want to completely separate our infrastructure from React Native and provide an experience most JavaScript developers are familiar with, namely the node module format. We want to even go further, and let you choose your own packager and asset pipeline or even integrate into your existing infrastructure.

React Native users need not to understand how the packager work, however, this documentation might be useful for advanced users and people who want to fix bugs or add features to the packager (patches welcome!).

HTTP interface

The main way you'd interact with the packager is via the HTTP interface. The following is the list of endpoints and their respective functions.

/path/to/moduleName.bundle

Does the following in order:

• parse out path/to/moduleName
• add a .js suffix to the path
• looks in your project root(s) for the file
• recursively collects all the dependencies from an in memory graph
• runs the modules through the transformer (might just be cached)
• concatenate the modules' content into a bundle
• responds to the client with the bundle (and a SourceMap URL)

/path/to/moduleName.map

• if the package has been previously generated via the .bundle endpoint then the source map will be generated from that package
• if the package has not been previously asked for, this will go through the same steps outlined in the .bundle endpoint then generate the source map.

Note that source map generation currently assumes that the code has been compiled with jstransform, which preserves line and column numbers which allows us to generate source maps super fast.

/path/to/moduleName.(map|bundle) query params

You can pass options for the bundle creation through the query params, if the option is boolean 1/0 or true/false is accepted.

Here are the current options the packager accepts:

• dev boolean, defaults to true: sets a global __DEV__ variable which will effect how the React Native core libraries behave.
• minify boolean, defaults to false: whether to minify the bundle.
• runModule boolean, defaults to true: whether to require your entry point module. So if you requested moduleName, this option will add a require('moduleName') the end of your bundle.
• inlineSourceMap boolean, defaults to false: whether to inline source maps.

/debug

This is a page used for debugging, it offers a link to a single page :

• Cached Packages: which shows you the packages that's been already generated and cached

Programmatic API

The packager is made of two things:

• The core packager (which we're calling ReactPackager)
• The scripts, devtools launcher, server run etc.

ReactPackager is how you mainly interact with the API.

var ReactPackager = require('path/to/packager/');

ReactPackager.buildBundle(serverOptions, bundleOptions)

Builds a bundle according to the provided options.

serverOptions

• projectRoots array (required): Is the roots where your JavaScript file will exist
• blacklistRE regexp: Is a pattern to ignore certain paths from the packager
• polyfillModuleName array: Paths to polyfills you want to be included at the start of the bundle
• cacheVersion string: used in creating the cache file
• resetCache boolean, defaults to false: whether to use the cache on disk
• transformModulePath string: Path to the module used as a JavaScript transformer
• nonPersistent boolean, defaults to false: Whether the server should be used as a persistent deamon to watch files and update itself
• getTransformOptions function: A function that acts as a middleware for generating options to pass to the transformer based on the bundle being built.
• reporter object (required): An object with a single function update that is called when events are happening: build updates, warnings, errors.

bundleOptions

• entryFile string (required): the entry file of the bundle, relative to one of the roots.
• dev boolean (defaults to true): sets a global __DEV__ variable which will effect how the React Native core libraries behave.
• minify boolean: Whether to minify code and apply production optimizations.
• runModule boolean (defaults to true): whether to require your entry point module.
• inlineSourceMap boolean, defaults to false: whether to inline source maps.
• platform string: The target platform for the build
• generateSourceMaps boolean: Whether to generate source maps.
• sourceMapUrl string: The url of the source map (will be appended to the bundle).

Debugging

To get verbose output when running the packager, define an environment variable:

export DEBUG=RNP:*

You can combine this with other values, e.g. DEBUG=babel,RNP:*. Under the hood this uses the debug package, see its documentation for all the available options.

The /debug endpoint discussed above is also useful.

FAQ

Can I use this in my own non-React Native project?

Yes. It's not really tied to React Native, however feature development is informed by React Native needs.

Why didn't you use webpack?

We love webpack, however, when we tried on our codebase it was slower than our developers would like it to be. You can find more discussion about the subject here.