browser shim for window.import()

import.js

window.import = function (transformer) {
	const ROOT = location.origin;
	const REQUIRE_CACHE = new Map();
	const RE_EXT = /\.js\w*$/;

	function fqn(url, dirname) {
		const {href} = new URL(url.startsWith('http') ? url : (url.startsWith('/') ? ROOT : dirname) + url),
			last = href.lastIndexOf('/'), dir = href.slice(0, last+1), name = href.slice(last+1),
			filename = name ? (RE_EXT.test(name) ? name: name+'.js') : 'index.js';
		return {dir, abs: dir+filename};
	}

	function require(url) {
		if (REQUIRE_CACHE.has(url)) {
			return Promise.resolve(REQUIRE_CACHE.get(url).exports);
		}
		const {dir, abs} = fqn(url, this.__dirname);

		if (REQUIRE_CACHE.has(abs)) {
			return Promise.resolve(REQUIRE_CACHE.get(abs).exports);
		}
		if (abs.endsWith('.json')) {
			return fetch(abs).then(r => r.json()).then(code => {
				const MODULE = {exports: code};
				REQUIRE_CACHE.set(abs, MODULE);
				return MODULE;
			});
		}
		return fetch(abs).then(r => r.text())
			.then(code => {
				const executableCode = Function('module', 'exports', '__dirname', 'require', code + '\n\n//# sourceURL=' + abs);
				const MODULE = {
					__dirname: dir,
					exports: {}
				};
				REQUIRE_CACHE.set(abs, MODULE);
				executableCode.call(MODULE.exports, MODULE, MODULE.exports, dir, require.bind(MODULE));
				return MODULE.exports;
			});
	}
	return Object.assign(require.bind({__dirname: ROOT}), {cache: REQUIRE_CACHE});
}();

// ex: window.import('https://unpkg.com/js-md5').then(md5 => console.log(md5('test')))

package.json

{
  "name": "window.import",
  "version": "1.0.1",
  "description": "browser shim for window.import()",
  "repository": "https://gist.github.com/caub/458cfe944f8abcf7b1aec608d0a878cc",
  "main": "import.js",
  "author": "caub",
  "license": "ISC",
  "homepage": "https://gist.github.com/caub/458cfe944f8abcf7b1aec608d0a878cc"
}

~README.md

install with npm i window.import or npm i gist:458cfe944f8abcf7b1aec608d0a878cc