Detect yarn usage from execpath env variable as signal to use yarn

as the package manager. Otherwise it will use npm.

packages/create-react-app/createReactApp.js

@@ -49,6 +49,14 @@ const validateProjectName = require('validate-npm-package-name');
 
 const packageJson = require('./package.json');
 
+function isUsingYarn() {
+  // Yarn 1 and 2 set npm_execpath to a path ending with 'yarn' so we can detect
+  // that to determine if the script was run using yarn
+  // https://github.com/yarnpkg/yarn/blob/v1.22.11/src/util/execute-lifecycle-script.js#L81
+  // https://github.com/yarnpkg/berry/blob/%40yarnpkg/core%2F3.1.0-rc.2/packages/yarnpkg-core/sources/scriptUtils.ts#L113
+  return path.basename(process.env.npm_execpath || '', '.js') === 'yarn';
+}
+
 let projectName;
 
 function init() {
@@ -69,7 +77,7 @@ function init() {
       '--template <path-to-template>',
       'specify a template for the created project'
     )
-    .option('--use-npm')
+    .option('--use-yarn', 'use yarn as the package manager', isUsingYarn())
     .option('--use-pnp')
     .allowUnknownOption()
     .on('--help', () => {
@@ -228,14 +236,14 @@ function init() {
           program.verbose,
           program.scriptsVersion,
           program.template,
-          program.useNpm,
+          program.useYarn,
           program.usePnp
         );
       }
     });
 }
 
-function createApp(name, verbose, version, template, useNpm, usePnp) {
+function createApp(name, verbose, version, template, useYarn, usePnp) {
   const unsupportedNodeVersion = !semver.satisfies(
     // Coerce strings with metadata (i.e. `15.0.0-nightly`).
     semver.coerce(process.version),
@@ -276,7 +284,6 @@ function createApp(name, verbose, version, template, useNpm, usePnp) {
     JSON.stringify(packageJson, null, 2) + os.EOL
   );
 
-  const useYarn = useNpm ? false : shouldUseYarn();
   const originalDirectory = process.cwd();
   process.chdir(root);
   if (!useYarn && !checkThatNpmCanReadCwd()) {
@@ -351,15 +358,6 @@ function createApp(name, verbose, version, template, useNpm, usePnp) {
   );
 }
 
-function shouldUseYarn() {
-  try {
-    execSync('yarnpkg --version', { stdio: 'ignore' });
-    return true;
-  } catch (e) {
-    return false;
-  }
-}
-
 function install(root, useYarn, usePnp, dependencies, verbose, isOnline) {
   return new Promise((resolve, reject) => {
     let command;

tasks/e2e-installs.sh

@@ -112,10 +112,11 @@ cd "$temp_app_path"
 npx create-react-app test-app-dist-tag --scripts-version=@latest
 cd test-app-dist-tag
 
-# Check corresponding scripts version is installed and no TypeScript is present.
+# Check corresponding scripts version is installed and no TypeScript or yarn is present by default
 exists node_modules/react-scripts
 ! exists node_modules/typescript
 ! exists src/index.tsx
+! exists yarn.lock
 exists src/index.js
 checkDependencies
 
@@ -133,16 +134,16 @@ grep '"version": "1.0.17"' node_modules/react-scripts/package.json
 checkDependencies
 
 # ******************************************************************************
-# Test --use-npm flag
+# Test --use-yarn flag
 # ******************************************************************************
 
 cd "$temp_app_path"
-npx create-react-app test-use-npm-flag --use-npm --scripts-version=1.0.17
-cd test-use-npm-flag
+npx create-react-app test-use-yarn-flag --use-yarn --scripts-version=1.0.17
+cd test-use-yarn-flag
 
 # Check corresponding scripts version is installed.
 exists node_modules/react-scripts
-[ ! -e "yarn.lock" ] && echo "yarn.lock correctly does not exist"
+exists yarn.lock
 grep '"version": "1.0.17"' node_modules/react-scripts/package.json
 checkDependencies
 
@@ -277,7 +278,7 @@ npm start -- --smoke-test
 # Test when PnP is enabled
 # ******************************************************************************
 cd "$temp_app_path"
-npx create-react-app test-app-pnp --use-pnp
+npx create-react-app test-app-pnp --use-yarn --use-pnp
 cd test-app-pnp
 ! exists node_modules
 exists .pnp.js

tasks/e2e-kitchensink-eject.sh

@@ -110,9 +110,6 @@ cd "$temp_app_path/test-kitchensink"
 # In kitchensink, we want to test all transforms
 export BROWSERSLIST='ie 9'
 
-# Link to test module
-npm link "$temp_module_path/node_modules/test-integrity"
-
 # ******************************************************************************
 # Finally, let's check that everything still works after ejecting.
 # ******************************************************************************
@@ -124,9 +121,6 @@ echo yes | npm run eject
 rm yarn.lock
 yarn add @babel/plugin-transform-react-jsx-source @babel/plugin-syntax-jsx @babel/plugin-transform-react-jsx @babel/plugin-transform-react-jsx-self
 
-# Link to test module
-npm link "$temp_module_path/node_modules/test-integrity"
-
 # Test the build
 REACT_APP_SHELL_ENV_MESSAGE=fromtheshell \
   NODE_PATH=src \