Customizing Metro

When you run expo start, the CLI uses Metro to bundle JavaScript for Android and iOS platforms. By default Expo CLI will use the Metro configuration defined in the @expo/metro-config package. You can add custom options for Metro by creating a file named metro.config.js in the project root directory.
The metro.config.js file looks like this:
module.exports = {
  /* add options here */
};
It can also export a function or an async function, if necessary:
module.exports = async () => {
  return {
    /* dynamically created options can be added here */
  };
};
You can find a complete list of supported options in the Metro docs: Configuring Metro. Please note that you only need to specify the options that you want to customize: the custom config will be merged with the defaults from @expo/metro-config when using Expo CLI.
To add to an value, such as an array of file extensions, defined in the default configuration, you can access the defaults using the getDefaultConfig(projectRoot) function defined in @expo/metro-config. Add @expo/metro-config to the dependencies of your project to do this.

One use case for custom metro.config.js is adding more file extensions that are considered to be an asset. Many image, video, audio and font formats (e.g. jpg, png, mp4, mp3 and ttf) are included by default. To add more asset file extensions, create a metro.config.js file in the project root. In the file add the file extension (without a leading .) to assetExts:
const { getDefaultConfig } = require('@expo/metro-config');

const defaultConfig = getDefaultConfig(__dirname);

module.exports = {
  resolver: {
    assetExts: [...defaultConfig.resolver.assetExts, 'db'],
  },
};
(This example adds .db, the extension of SQLite database files to assetExts).