Fix #448 - Add offline build detection and advice (#452)

src/cli.js

@@ -4,6 +4,7 @@ import 'source-map-support/register';
 import program from 'commander';
 import nativefier from './index';
 
+const dns = require('dns');
 const packageJson = require('./../package');
 
 function collect(val, memo) {
@@ -23,6 +24,15 @@ function getProcessEnvs(val) {
   return pEnv;
 }
 
+function checkInternet() {
+  dns.lookup('npmjs.com', (err) => {
+    if (err && err.code === 'ENOTFOUND') {
+      console.log('\nNo Internet Connection\nTo offline build, download electron from https://github.com/electron/electron/releases\nand place in ~/AppData/Local/electron/Cache/ on Windows,\n~/.cache/electron on Linux or ~/Library/Caches/electron/ on Mac\nUse --electron-version to specify the version you downloaded.');
+    }
+  });
+}
+
+
 if (require.main === module) {
   program
     .version(packageJson.version)
@@ -77,7 +87,7 @@ if (require.main === module) {
   if (!process.argv.slice(2).length) {
     program.help();
   }
-
+  checkInternet();
   nativefier(program, (error, appPath) => {
     if (error) {
       console.error(error);