index.html

<meta charset="UTF-8">
<meta property="og:type" content="website" />
<meta property="og:title" content="{% if amount %}Pay {{amount}}{% else %}Pay me{% endif %}" />
<meta property="og:image" content="{{qrUrl}}">
<meta property="og:url" content="{{url}}">
<meta property="og:description" content="">
<meta name="viewport" content="width=device-width,user-scalable=no,initial-scale=1.0">
<title>{% if amount %}Pay {{amount}}{% else %}Pay me{% endif %}</title>
<style>
body{
	margin: 0; padding: 0;
	background-color: #333;
	color: #333;
	font-family: -apple-system,system-ui,BlinkMacSystemFont,"Segoe UI",Roboto,"Helvetica Neue",Arial,sans-serif;
}
#inner{
	width: 100%;
	max-width: 400px;
	margin: auto;
	margin-top: 20px;
	background-color: white;
	padding: 10px;
	box-sizing: border-box;
	text-align: center;
	font-size: 9pt;
	box-shadow: #222 0px 3px 30px;
}
img{
	width: 100%;
	image-rendering: pixelated;
}
#ref{
	cursor: pointer;
}
</style>
<div id="inner">
	<img src="{{qr}}">
	<p>Ref ID: <span id="ref">{{id}}</span></p>
	{% if amount %}<p>Amount: {{amount}}</p>{% endif %}
</div>
<script>
	document.getElementById("ref").addEventListener("click", function(){
		window.getSelection().selectAllChildren(this);
		document.execCommand("copy");
	});
</script>

index.js

const Koa = require('koa');
const Router = require('koa-trie-router');
const nunjucks = require('nunjucks');
const promptpay = require('promptpay-qr');
const qrcode = require('qrcode');
const stream = require('stream');

const PROMPTPAY_IDS = {
	k: '004999000071927',
};
const DEFAULT_ID = process.env['DEFAULT_ID'] || 'k';

const app = new Koa();
app.proxy = true;
const router = new Router();
nunjucks.configure('views', { autoescape: true });

router.use((ctx, next) => {
	if (process.env.NODE_ENV === 'production') {
		ctx.set('Cache-Control', 'public,max-age=2592000');
	}
	return next();
});

router.get('/', async (ctx, next) => {
	await response(ctx, {
		id: PROMPTPAY_IDS[DEFAULT_ID],
	});
	return next();
});
router.get('/:amount(\\d{1,10}(\\.\\d{2}|))', async (ctx, next) => {
	await response(ctx, {
		id: PROMPTPAY_IDS[DEFAULT_ID],
		amount: parseFloat(ctx.params.amount),
	});
	return next();
});
router.get('/:account', async (ctx, next) => {
	let id = PROMPTPAY_IDS[ctx.params.account];
	if (!id) {
		return next();
	}
	await response(ctx, {
		id: id,
	});
	return next();
});
router.get('/:account/:amount(\\d{1,10}(\\.\\d{2}|))', async (ctx, next) => {
	let id = PROMPTPAY_IDS[ctx.params.account];
	if (!id) {
		return next();
	}
	await response(ctx, {
		id: id,
		amount: parseFloat(ctx.params.amount),
	});
	return next();
});

// qr
router.get('/_qr/', async (ctx, next) => {
	await qrResponse(ctx, {
		id: PROMPTPAY_IDS[DEFAULT_ID],
	});
	return next();
});
router.get('/_qr/:amount(\\d{1,10}(\\.\\d{2}|))', async (ctx, next) => {
	await qrResponse(ctx, {
		id: PROMPTPAY_IDS[DEFAULT_ID],
		amount: parseFloat(ctx.params.amount),
	});
	return next();
});
router.get('/_qr/:account', async (ctx, next) => {
	let id = PROMPTPAY_IDS[ctx.params.account];
	if (!id) {
		return next();
	}
	await qrResponse(ctx, {
		id: id,
	});
	return next();
});
router.get(
	'/_qr/:account/:amount(\\d{1,10}(\\.\\d{2}|))',
	async (ctx, next) => {
		let id = PROMPTPAY_IDS[ctx.params.account];
		if (!id) {
			return next();
		}
		await qrResponse(ctx, {
			id: id,
			amount: parseFloat(ctx.params.amount),
		});
		return next();
	}
);

async function response(ctx, data) {
	ctx.set('Content-Type', 'text/html');
	let payload = promptpay(data.id, data);
	let qr = await qrcode.toDataURL(payload, {
		errorCorrectionLevel: 'H',
		scale: 1,
		dark: '#333333ff',
	});
	ctx.body = nunjucks.render('index.html', {
		id: data.id,
		amount: data.amount,
		qr: qr,
		qrUrl: `${ctx.origin}/_qr${ctx.path}`,
		url: ctx.href,
	});
}

async function qrResponse(ctx, data) {
	ctx.set('Content-Type', 'image/png');
	ctx.status = 200;
	let payload = promptpay(data.id, data);
	let scale = parseInt(ctx.query.scale, 10);

	if (isNaN(scale)) {
		scale = 4;
	}

	scale = Math.min(Math.max(scale, 4), 100);

	let inOutStream = new stream.Transform({
		transform(chunk, encoding, callback) {
			this.push(chunk);
			callback();
		},
	});

	qrcode.toFileStream(inOutStream, payload, {
		errorCorrectionLevel: 'H',
		scale: scale,
	});

	inOutStream.pipe(ctx.res);

	return new Promise((resolve) => {
		inOutStream.on('finish', resolve);
	});
}

app.use(router.middleware());

app.listen(3000);
console.log('http://localhost:3000');

package.json

{
  "main": "index.js",
  "dependencies": {
    "koa": "^2.5.2",
    "koa-trie-router": "^2.1.6",
    "nunjucks": "^3.1.3",
    "promptpay-qr": "^0.4.4",
    "qrcode": "^1.2.0"
  }
}