【React】環境構築のエラーメモ（npx create-react-app）

## はじめに
Reactの環境構築でESLint関係のエラーが発生し、解決に少し苦戦したため、共有致します。
経緯は、Reactの環境構築の前にJSの開発環境を構築（今よりも良く）するために`Prettier`（コードフォーマット）と`ESLint`（静的解析）を導入したことが始まりでした。

## 私が行ったESLint導入（あくまでも状況を再現するためですので、スキップして頂いても構いません）
事前にVScodeで下記プラグインをインストール

- [Prettier](https://marketplace.visualstudio.com/items?itemName=esbenp.prettier-vscode)
- [ESLint](https://marketplace.visualstudio.com/items?itemName=dbaeumer.vscode-eslint)

次に下記コマンドを叩く

```
npm install -D eslint prettier  //eslintとprettierをインストール
npm install -D eslint-config-prettier  //eslintとprettierの競合をいい感じに調整してくれる設定
```

## 環境構築
プロジェクトのディレクトリに移動して下記コマンドを叩く

```
npx create-react-app my-app
cd my-app
npm start
```

`npm start`を叩いたときに、下記のようなエラーが発生

## エラー内容
```
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:

  "eslint": "5.12.0"

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

  /Users/myname/Works/React/myfolder/node_modules/eslint (version: 5.15.3) 

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 "eslint" 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 /Users/myname/Works/React/myfolder/node_modules/eslint is outside your project directory.
     For example, you might have accidentally installed something in your home folder.

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

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! tutorial@0.1.0 start: `react-scripts start`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the tutorial@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:
```

## 解決方法
ログの中にある下記の1~4を順番に行いましたが、解決しませんでした。

```
  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 "eslint" 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.
```

翻訳を通してよく読んでみる、、

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

訳すと

```
ツリーの上位で異なるバージョンのeslintが検出されました。
手動でインストールしようとしないでください。パッケージマネージャーが自動的にインストールします。
```

僕「ん、、なんか見覚えのあることで怒られてる、、」
僕「あー、、あのとき手動でインストールしたESLintかも」
僕「**パッケージマネージャーが自動的にインストール**してくれるらしいから手動で入れたESLintは削除するか」

下記コマンドを叩く
（Prettierは関係なさそうだったので削除しませんでした）

```
npm uninstall eslint
npm uninstall eslint-config-prettier
```

これで無事解決。
エラー解決の助けになれば幸いです。