dns: lookupService() callback must be a function

lookupService() requires a callback function. This commit adds
a check to verify that the callback is actually a function.

PR-URL: https://github.com/nodejs/node/pull/8170
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>

lib/dns.js

@@ -191,9 +191,12 @@ exports.lookupService = function(host, port, callback) {
   if (isIP(host) === 0)
     throw new TypeError('"host" argument needs to be a valid IP address');
 
-  if (port == null || !isLegalPort(port))
+  if (!isLegalPort(port))
     throw new TypeError(`"port" should be >= 0 and < 65536, got "${port}"`);
 
+  if (typeof callback !== 'function')
+    throw new TypeError('"callback" argument must be a function');
+
   port = +port;
   callback = makeAsync(callback);
 

test/parallel/test-dns.js

@@ -177,3 +177,7 @@ assert.throws(function() {
 assert.throws(function() {
   dns.lookupService('0.0.0.0', 'test', noop);
 }, /"port" should be >= 0 and < 65536, got "test"/);
+
+assert.throws(() => {
+  dns.lookupService('0.0.0.0', 80, null);
+}, /^TypeError: "callback" argument must be a function$/);