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Bundling API

Quick Start​

  • Compile a file

    const config = await Metro.loadConfig();

    await Metro.runBuild(config, {
    entry: 'index.js',
    out: 'bundle.js',
  • Run a server and watch the filesystem for changes

    const config = await Metro.loadConfig();

    await Metro.runServer(config);
  • Create a Connect middleware and plug it into a server

    const Metro = require('metro');
    const express = require('express');
    const app = express();
    const server = require('http').Server(app);

    Metro.loadConfig().then(async config => {
    const connectMiddleware = await Metro.createConnectMiddleware(config);
    const {server: {port}} = config;



All functions exposed below accept an additional config option. This object should be the Metro configuration exposed by your metro.config.js file - you can obtain it using Metro.loadConfig.


Basic options: config, cwd

Load the Metro configuration, either from config in options if specified, or by traversing the directory hierarchy from cwd to the root until it finds a file (by default metro.config.js). The returned configuration will have been normalized and merged with Metro's default values.

async runMetro(config)​

Creates a Metro server based on the config and returns it. You can use this as a middleware in your existing server.

async runBuild(config, <options>)​

Required options: entry, out

Basic options: dev, minify, platform, sourceMap, sourceMapUrl

Bundles entry for the given platform, and saves it to location out. If sourceMap is set, also generates a source map. The source map will be inlined, unless sourceMapUrl is also defined. In the latter case, a new file will be generated with the basename of the sourceMapUrl parameter.

async runServer(config, <options>)​

Basic options: host, port, secureServerOptions, secure (DEPRECATED), secureKey (DEPRECATED), secureCert (DEPRECATED)

Starts a full Metro HTTP server. It will listen on the specified host:port, and can then be queried to retrieve bundles for various entry points. If the secureServerOptions family of options are present, the server will be exposed over HTTPS.

secure, secureKey, secureCert are now deprecated and will be removed in a later release. The presence of secureServerOptions, along with its options will make Metro run over https.

createConnectMiddleware(config, <options>)​

Basic options: port, onBundleBuilt

Instead of creating the full server, creates a Connect middleware that answers to bundle requests. This middleware can then be plugged into your own servers. The port parameter is optional and only used for logging purposes. The onBundleBuilt function is optional, is passed the bundle name, and is called when the server has finishing creating the bundle.