fix(create): make --packageManager flag work (#3498)

Author: mgred

packages/create/index.js

@@ -75,7 +75,15 @@ function main(argv, error = console.error, log = console.log) {
   }
 
   // Which package manager will be used in the new project
-  const pkgMgr = args['packageManager'] || detectRunningUnderYarn() ? 'yarn' : 'npm';
+  let pkgMgr = args['packageManager'];
+
+  if (!pkgMgr) {
+    pkgMgr = detectRunningUnderYarn() ? 'yarn' : 'npm';
+  } else if(pkgMgr !== 'yarn' && pkgMgr !== 'npm') {
+    error('Please select between \'yarn\' and \'npm\' when providing --packageManager');
+    usage(error);
+    return 1;
+  }
 
   log_verbose('Running with', process.argv);
   log_verbose('Environment', process.env);

packages/create/test.js

@@ -70,6 +70,16 @@ wkspContent = read('default_to_yarn/WORKSPACE');
 if (wkspContent.indexOf('yarn_install(') < 0) {
   fail('should use yarn by default');
 }
+
+exitCode = main(['use_npm_with_yarn', '--packageManager=npm']);
+if (exitCode != 0) fail('should be success');
+wkspContent = read('use_npm_with_yarn/WORKSPACE');
+if (wkspContent.indexOf('npm_install(') < 0) {
+  fail('should use npm as selected');
+}
+
+exitCode = main(['neither_yarn_nor_npm', '--packageManager=foo']);
+if (exitCode != 1) fail('should exit 1 when selecting neither \'yarn\' nor \'npm\'');
 // TODO: run bazel in the new directory to verify a build works
 
 exitCode = main(['--typescript', 'with_ts'], captureError);