Closes prisma/prisma-client-js#561 and prisma/prisma-client-js#560 Us…

…e npm by default, use yarn only if there is a yarn.lock

cli/sdk/package.json

@@ -18,7 +18,7 @@
     "scripts"
   ],
   "prisma": {
-    "version": "377df4fe30aa992f13f1ba152cf83d5770bdbc85"
+    "version": "4ff8379527ec7797e7bb5b55d374f82f5812a6f9"
   },
   "devDependencies": {
     "@types/jest": "24.0.22",
@@ -49,6 +49,7 @@
     "execa": "^3.4.0",
     "flat-map-polyfill": "^0.3.8",
     "globby": "^9.2.0",
+    "has-yarn": "^2.1.0",
     "make-dir": "^3.0.0",
     "node-fetch": "2.6.0",
     "p-map": "^3.0.0",
@@ -79,4 +80,4 @@
       "git add"
     ]
   }
-}
+}

cli/sdk/src/predefinedGeneratorResolvers.ts

@@ -1,5 +1,6 @@
 import resolvePkg from 'resolve-pkg'
 import chalk from 'chalk'
+import hasYarn from 'has-yarn'
 import execa from 'execa'
 import path from 'path'
 import fs from 'fs'
@@ -108,9 +109,9 @@ Please try to install it with ${chalk.bold.greenBright(
 }
 
 async function installPackage(baseDir: string, pkg: string): Promise<void> {
-  const yarnInstalled = await isYarnInstalled()
+  const yarnUsed = hasYarn(baseDir)
 
-  const cmdName = yarnInstalled ? 'yarn add' : 'npm install'
+  const cmdName = yarnUsed ? 'yarn add' : 'npm install'
 
   await execa.command(`${cmdName} ${pkg}`, {
     cwd: baseDir,
@@ -121,12 +122,3 @@ async function installPackage(baseDir: string, pkg: string): Promise<void> {
     },
   })
 }
-
-async function isYarnInstalled(): Promise<boolean> {
-  try {
-    await execa.command(`yarn --version`, { stdio: `ignore` })
-    return true
-  } catch (err) {
-    return false
-  }
-}