nomodule.js

// Mode :: "nomodule"
// Meant for browsers that
// do not support <script type=module /> and
// do not support any static `import` statements.

import imports from 'rewrite-imports';

var CACHE = {};

function run(url, str) {
	window.dimport = dimport;

	var key, keys=[], urls=[], mod={ exports:{} };

	var txt = imports(
			// Ensure full URLs & Gather static imports
			str.replace(/(^|\s|;)(import\s*)(\(|.*from\s*|)['"]([^'"]+)['"];?/gi, function (_, pre, req, type, loc) {
				loc = "'" + new URL(loc, url).href + "'";
				return pre + req + type + (type == '(' ? loc : `'$dimport[${urls.push(loc) - 1}]';`);
			})

			// Ensure we caught all dynamics (multi-line clause)
			.replace(/(^|\s|;)(import)(?=\()/g, '$1window.dimport')

			// Exports
			.replace(/export default/, 'module.exports =')
			.replace(/export\s+(const|function|class|let|var)\s+(.+?)(?=(\(|\s|=))/gi, function (_, type, name) {
				return keys.push(name) && (type + ' ' + name);
			})
			.replace(/export\s*\{([\s\S]*?)\}/gi, function (_, list) {
				var tmp, out='', arr=list.split(',');
				while (tmp = arr.shift()) {
					tmp = tmp.trim().split(' as ');
					out += 'exports.' + (tmp[1] || tmp[0]) + ' = ' + tmp[0] + ';\n';
				}
				return out;
			})
	, 'eval');

	for (keys.sort(); key = keys.shift();) {
		txt += '\nexports.' + key + ' = ' + key + ';';
	}

	return Promise.resolve(
		new Function('module', 'exports',
			urls.length
				? 'return Promise.all([' + urls.join() + '].map(window.dimport)).then(function($dimport){' + txt + '});'
				: txt
		)(mod, mod.exports)
	).then(function () {
		mod.exports.default = mod.exports.default || mod.exports;
		return mod.exports;
	});
}

function dimport(url) {
	url = new URL(url, location.href).href;

	try {
		return new Function("return import('" + url + "')").call();
	} catch (err) {
		return CACHE[url]
			? Promise.resolve(CACHE[url])
			: fetch(url).then(function (r) {
					return r.text();
				})
				.then(run.bind(run, url))
				.then(function (x) {
					return CACHE[url] = x;
				});
	}
}

// Runtime hookups
var tag = document !== void 0 && document.currentScript || document.querySelector('script[data-main]');
if (tag) tag.text ? run(location.href, tag.text) : (tag=tag.getAttribute('data-main')) && dimport(tag);

export default dimport;