Check for yarn.lock while isntalling

Author: reznord

src/lib/setup.js

@@ -1,3 +1,5 @@
+import path from 'path';
+import fs from 'fs.promised';
 import spawn from 'cross-spawn-promise';
 import { commandExists } from './shell';
 
@@ -16,6 +18,12 @@ const install = async (yarn, cwd, packages, env) => {
 	let isYarnAvailable = await commandExists('yarn');
 	let toInstall = packages.filter(Boolean);
 
+	// pass null to use yarn only if yarn.lock is present
+	if (!yarn) {
+		try { yarn = await fs.stat(path.resolve(cwd, 'yarn.lock')).isFile(); }
+		catch (e) { yarn = false; }
+	}
+
 	if (isYarnAvailable && yarn) {
 		let args = ['add'];
 		if (isDev) {