*resolve-export-map.md

resolve-export-map

import { resolveExportMap } from 'resolve-export-map';

const demoMeta = {
  exports: {
    '.': './index.mjs',
    './foo': {
      import: './dist/foo.mjs',
      require: './dist/foo.cjs'
    },
    './bar': {
      browser: './bar/index.umd.js' // only used if "browser" env key is passed
    },
    './bar/': './bar/' // enable disk resolution within this dir
  }
};


resolveExportMap(demoMeta, '.', []) // './index.mjs'

resolveExportMap(demoMeta, './foo', []) // false  (no disk lookup allowed)
resolveExportMap(demoMeta, './foo', ['import']) // './dist/foo.mjs'
resolveExportMap(demoMeta, './foo', ['require', 'default']) // './dist/foo.mjs'

resolveExportMap(demoMeta, './bar', ['browser']) // './bar/index.umd.js'
resolveExportMap(demoMeta, './bar/test.js', []) // true  (disk lookup allowed)

resolve-export-map.js

/** Get the best resolution for an entry from an Export Map
 * @param {Object} exp `package.exports`
 * @param {string} entry `./foo` or `.`
 * @param {string[]} [envKeys=ENV_KEYS] package environment keys
 * @returns {string | boolean} a resolved path, or a boolean indicating if the given entry is exposed
 */
export function resolveExportMap(exp, entry, envKeys) {
	if (typeof exp === 'string') {
		// {"exports":"./foo.js"}
		// {"exports":{"./foo":"./foo.js"}}
		return exp;
	}
	let isFileListing;
	let isDirectoryExposed = false;
	for (let i in exp) {
		if (isFileListing === undefined) isFileListing = i[0] === '.';
		if (isFileListing) {
			// {"exports":{".":"./index.js"}}
			if (i === entry) {
				return resolveExportMap(exp[i], entry, envKeys);
			}
			if (!isDirectoryExposed && i.endsWith('/') && entry.startsWith(i)) {
				isDirectoryExposed = true;
			}
		} else if (envKeys.includes(i)) {
			// {"exports":{"import":"./foo.js"}}
			return resolveExportMap(exp[i], entry, envKeys);
		}
	}
	return isDirectoryExposed;
}

Thanks @developit :)
Your code snippet doesn't quite work for me (failed resolutions), but Lukeed's tiny NPM package has served me well: https://github.com/lukeed/resolve.exports