http: add type checking for hostname

Add type checking for options.hostname / options.host
Maintains the use of `undefined` and `null` for default `localhost`,
but other falsy values (like `false`, `0` and `NaN`) are rejected.

lib/_http_client.js

@@ -65,6 +65,14 @@ function isInvalidPath(s) {
   return false;
 }
 
+function validateHost(host, name) {
+  if (host != null && typeof host !== 'string') {
+    throw new TypeError(
+      `"options.${name}" must either be a string, undefined or null`);
+  }
+  return host;
+}
+
 function ClientRequest(options, cb) {
   OutgoingMessage.call(this);
 
@@ -123,7 +131,8 @@ function ClientRequest(options, cb) {
                       this.agent && this.agent.defaultPort;
 
   var port = options.port = options.port || defaultPort || 80;
-  var host = options.host = options.hostname || options.host || 'localhost';
+  var host = options.host = validateHost(options.hostname, 'hostname') ||
+                            validateHost(options.host, 'host') || 'localhost';
 
   var setHost = (options.setHost === undefined);
 

test/parallel/test-http-hostname-typechecking.js

@@ -0,0 +1,27 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+// All of these values should cause http.request() to throw synchronously
+// when passed as the value of either options.hostname or options.host
+const vals = [{}, [], NaN, Infinity, -Infinity, true, false, 1, 0, new Date()];
+
+function errCheck(name) {
+  return new RegExp(`^TypeError: "options\\.${name}" must either be a ` +
+                    'string, undefined or null$');
+}
+
+vals.forEach((v) => {
+  assert.throws(() => http.request({hostname: v}), errCheck('hostname'));
+  assert.throws(() => http.request({host: v}), errCheck('host'));
+});
+
+// These values are OK and should not throw synchronously
+['', undefined, null].forEach((v) => {
+  assert.doesNotThrow(() => {
+    http.request({hostname: v}).on('error', common.noop);
+    http.request({host: v}).on('error', common.noop);
+  });
+});