Node DNS caching with Promises

cache.js

var dns = require('dns'), cache = {};
dns._lookup = dns.lookup;

dns.lookup = function(domain) {
	return new Promise( (resolve, reject) => {
		var key = domain;
		if (key in cache) {
			var ip = cache[key], 
				ipv = ip.indexOf('.') !== -1 ? 4 : 6;

			return process.nextTick(function() { 
				resolve(ip);
			});
		}

		dns._lookup(domain, null, function(err, ip, ipv) {
			if (err) reject(err);
			cache[key] = ip;
			resolve(ip);
		});
	});
};

dns.lookup('google.com').then( (ip) => {
	console.log( ip );
}).catch( (err) => {
	console.error( err );
});