How to append extension in metro.config.js for Metro Bundler?

Found the answer on how to include the defaults here: https://stackoverflow.com/a/55118654/844907.

/**
 * Metro configuration for React Native
 * https://github.com/facebook/react-native
 *
 * @format
 */

// get defaults assetExts array
const defaultAssetExts = require("metro-config/src/defaults/defaults").assetExts;

module.exports = {
    transformer: {
        getTransformOptions: async () => ({
            transform: {
                experimentalImportSupport: false,
                inlineRequires: false,
            },
        }),
    },
    resolver: {
        assetExts: [
            ...defaultAssetExts, // <- array spreading defaults
            'md'
        ]
    }
};

Other way you could do this is use the same syntax as react-native-svg-transformer

1. Make metro.config.js asynchronous
2. call the getDefaultConfig
3. Pull off the assetExts key

Example

const { getDefaultConfig } = require('metro-config')

module.exports = (async () => {
    const {
        resolver: { assetExts },
    } = await getDefaultConfig()
    return {
        transformer: {
            getTransformOptions: async () => ({
                transform: {
                    experimentalImportSupport: false,
                    inlineRequires: false,
                },
            }),
        },
        resolver: {
            assetExts: [...assetExts, 'md'],
        },
    }
})()