Socks5 proxy server in pure javascript

socks5.js

const net = require('net')
net.createServer(client => {
	client.once('data', data => {
		client.write(Buffer.from([5, 0]));

		client.once('data', data => {
			data = [...data];
			let ver = data.shift();
			let cmd = data.shift(); //1: connect, 2: bind, 3: udp
			let rsv = data.shift();
			let atyp = data.shift(); //1: ipv4(4bytes followed), 3: domain, 4: ipv6(16 bytes followed)
			let dstAddr, dstPort, serverStr = '';
			if (atyp === 1) {
				dstAddr = data.splice(0, 4);
				serverStr = dstAddr.join('.');
			} else if (atyp === 3) {
				let domainLength = data.shift();
				dstAddr = data.splice(0, domainLength);
				serverStr = Buffer.from(dstAddr).toString();
				dstAddr.unshift(domainLength);
			} else if (atyp === 4) {
				dstAddr = data.splice(0, 16);
				serverStr = [];
				for(let i = 0; i < 8; i++) {
					serverStr.push(Buffer.from(dstAddr.slice(i * 2, 2)).toString('hex'));
				}
				serverStr = serverStr.join(':');
			} else {
				client.destroy();
				return;
			}

			dstPort = data.splice(0, 2);
			const portNum = dstPort[0] * 256 + dstPort[1];

			const serverName = `${serverStr}:${portNum}`;
			console.log('>', serverName);
			
			const dstServer = net.connect({
				host: serverStr,
				port: portNum
			});

			dstServer.on('error', err => {
				console.log(`!Server Error(${serverName}):`, err.message);
				client.destroy();
			});

			client.on('error', err=>{
				console.log(`!Client Error(${serverName}):`, err.message);
				dstServer.destroy();
			});

			if (data && data.length > 0) {
				dstServer.write(Buffer.from(data));
			}

			client.write(Buffer.concat([
				Buffer.from([ver, 0, 0, atyp]),
				Buffer.from(dstAddr),
				Buffer.from(dstPort)
			]));

			dstServer.pipe(client);
			client.pipe(dstServer);
		});
	});
}).listen(8787, '127.0.0.1');