Improve check for used JS features (#544)

* Improve check for used JS features

- Moved check to ./lib/environment.js so it can be re-used in
  different modules
- Uses the vm module to check for the spread operator.
- Added `process.exit(1)` when the node environment is invalid.

lighthouse-cli/index.js

@@ -18,17 +18,17 @@
 
 'use strict';
 
+const environment = require('../lighthouse-core/lib/environment.js');
+if (!environment.checkNodeCompatibility()) {
+  console.warn('Compatibility error', 'Lighthouse requires node 5+ or 4 with --harmony');
+  process.exit(1);
+}
+
 const yargs = require('yargs');
-const semver = require('semver');
 const Printer = require('./printer');
 const lighthouse = require('../lighthouse-core');
 const log = require('../lighthouse-core/lib/log');
 
-// node 5.x required due to use of ES2015 features, like spread operator
-if (semver.lt(process.version, '5.0.0')) {
-  console.warn('Compatibility error', 'Lighthouse requires node 5+ or 4 with --harmony');
-}
-
 const cli = yargs
   .help('help')
   .version()

lighthouse-core/index.js

@@ -17,7 +17,12 @@
 
 'use strict';
 
-const semver = require('semver');
+const environment = require('../lighthouse-core/lib/environment');
+if (!environment.checkNodeCompatibility()) {
+  console.warn('Compatibility error', 'Lighthouse requires node 5+ or 4 with --harmony');
+  process.exit(1);
+}
+
 const Runner = require('./runner');
 const log = require('./lib/log.js');
 const ChromeProtocol = require('./gather/drivers/cri.js');
@@ -38,17 +43,14 @@ const Config = require('./config');
  *
  */
 
-// node 5.x required due to use of ES2015 features, like spread operator
-if (semver.lt(process.version, '5.0.0')) {
-  log.warn('Compatibility error', 'Lighthouse requires node 5+ or 4 with --harmony');
-}
-
 module.exports = function(url, flags, configJSON) {
   return new Promise((resolve, reject) => {
     if (!url) {
       return reject(new Error('Lighthouse requires a URL'));
     }
 
+    flags = flags || {};
+
     // set logging preferences, assume quiet
     flags.logLevel = flags.logLevel || 'error';
     log.setLevel(flags.logLevel);
@@ -59,8 +61,6 @@ module.exports = function(url, flags, configJSON) {
       log.warn('Lighthouse', 'Performance stats will be skewed redirecting from HTTP to HTTPS.');
     }
 
-    flags = flags || {};
-
     // Use ConfigParser to generate a valid config file
     const config = new Config(configJSON, flags.auditWhitelist);
 

lighthouse-core/lib/environment.js

@@ -0,0 +1,39 @@
+/**
+ * @license
+ * Copyright 2016 Google Inc. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+'use strict';
+
+const semver = require('semver');
+const vm = require('vm');
+
+function checkNodeCompatibility() {
+  // node 5.x required due to use of ES2015 features, like spread operator
+  if (semver.gte(process.version, '5.0.0')) {
+    return true;
+  }
+
+  try {
+    // Test for the availability of the spread operator.
+    new vm.Script('Math.max(...[ 5, 10 ])').runInThisContext();
+    return true;
+  } catch (e) {
+    return false;
+  }
+}
+
+module.exports = {
+  checkNodeCompatibility: checkNodeCompatibility
+};