Use npm as package manager by default

This removes the --use-npm flag and adds the --use-yarn flag.
It also removes the previous behavior of checking for the existence
of yarn as the signal to use it as the package manager.

Author: lukekarrys

packages/create-react-app/createReactApp.js

@@ -69,7 +69,7 @@ function init() {
       '--template <path-to-template>',
       'specify a template for the created project'
     )
-    .option('--use-npm')
+    .option('--use-yarn')
     .option('--use-pnp')
     .allowUnknownOption()
     .on('--help', () => {
@@ -228,14 +228,14 @@ 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 +276,6 @@ 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 +350,6 @@ 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