Reactの環境構築で詰まった時の対処法（yarn startが上手くいかない時）

参考にしたreactの環境構築サイトはこちら

　まず前提として自分の設定されているnodebrewやnpmのバージョン確認（使えるか確認）

$ node -v
v16.8.0

$ npm -v
7.21.0

$ yarn -v
1.22.11

普通に使えることを確認。
そしてyarn startしてみると以下のようなエラー発生

There might be a problem with the project dependency tree.
It is likely not a bug in Create React App, but something you need to fix locally.

The react-scripts package provided by Create React App requires a dependency:

  "webpack": "^4.22.0"

Don't try to install it manually: your package manager does it automatically.
However, a different version of webpack was detected higher up in the tree:

  /Users/testuser/node_modules/webpack (version: 5.37.0)

Manually installing incompatible versions is known to cause hard-to-debug issues.

If you would prefer to ignore this check, add SKIP_PREFLIGHT_CHECK=true to an .env file in your project.
That will permanently disable this message but you might encounter other issues.

To fix the dependency tree, try following the steps below in the exact order:

  1. Delete package-lock.json (not package.json!) and/or yarn.lock in your project folder.
  2. Delete node_modules in your project folder.
  3. Remove "webpack" from dependencies and/or devDependencies in the package.json file in your project folder.
  4. Run npm install or yarn, depending on the package manager you use.

In most cases, this should be enough to fix the problem.
If this has not helped, there are a few other things you can try:

  5. If you used npm, install yarn (http://yarnpkg.com/) and repeat the above steps with it instead.
     This may help because npm has known issues with package hoisting which may get resolved in future versions.

  6. Check if /home/testuser/node_modules/webpack is outside your project directory.
     For example, you might have accidentally installed something in your home folder.

  7. Try running npm ls webpack in your project folder.
     This will tell you which other package (apart from the expected react-scripts) installed webpack.

If nothing else helps, add SKIP_PREFLIGHT_CHECK=true to an .env file in your project.
That would permanently disable this preflight check in case you want to proceed anyway.

P.S. We know this message is long but please read the steps above :-) We hope you find them helpful!

npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! react-app@0.1.0 start: `react-scripts start`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the react-app@0.1.0 start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:

上記エラーの中に枝番で書かれてる解決方法を実践してみました（↓これです）

1. Delete package-lock.json (not package.json!) and/or yarn.lock in your project folder.
   （package-lock.jsonを削除 ）
2. Delete node_modules in your project folder.
   （node_modulesを削除）
3. Remove "webpack" from dependencies and/or devDependencies in the package.json file in your project folder.
   （package.jsonに記載のwebpackというオブジェクト？文言？を削除。ただこれは記述がなかったためスルーしました。）
4. Run npm install or yarn, depending on the package manager you use.
   （npmを再度インストール） これを試して再度yarn startしてみると

Compiled successfully!

You can now view helloworld in the browser.

  Local:            http://localhost:3000
  On Your Network:  http://*************

Note that the development build is not optimized.
To create a production build, use npm run build.

見事成功！！
しっかりと動きました！↓

事前に上階層のディレクトリにあるnode-moduleも消していたのでそれも試してみるといいかもしれませんね。