vm: consolidate validation

PR-URL: nodejs#18816
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/vm.js

@@ -79,22 +79,23 @@ Script.prototype.runInNewContext = function(sandbox, options) {
   return this.runInContext(context, options);
 };
 
+function validateString(prop, propName) {
+  if (prop !== undefined && typeof prop !== 'string')
+    throw new errors.TypeError('ERR_INVALID_ARG_TYPE', propName,
+                               'string', prop);
+}
+
 function getContextOptions(options) {
-  const contextOptions = options ? {
-    name: options.contextName,
-    origin: options.contextOrigin
-  } : {};
-  if (contextOptions.name !== undefined &&
-      typeof contextOptions.name !== 'string') {
-    throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'options.contextName',
-                               'string', contextOptions.name);
-  }
-  if (contextOptions.origin !== undefined &&
-      typeof contextOptions.origin !== 'string') {
-    throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'options.contextOrigin',
-                               'string', contextOptions.origin);
+  if (options) {
+    const contextOptions = {
+      name: options.contextName,
+      origin: options.contextOrigin
+    };
+    validateString(contextOptions.name, 'options.contextName');
+    validateString(contextOptions.origin, 'options.contextOrigin');
+    return contextOptions;
   }
-  return contextOptions;
+  return {};
 }
 
 let defaultContextNameIndex = 1;
@@ -120,10 +121,7 @@ function createContext(sandbox, options) {
       throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'options.name',
                                  'string', options.name);
     }
-    if (options.origin !== undefined && typeof options.origin !== 'string') {
-      throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'options.origin',
-                                 'string', options.origin);
-    }
+    validateString(options.origin, 'options.origin');
   } else {
     options = {
       name: `VM Context ${defaultContextNameIndex++}`