[Node][Yarn] create-react-app したプロジェクトでyarn start でエラー





# 起こったこと

react.js の`create-react-app` コマンドで作成した新規プロジェクトで
`yarn start` ( `react-scripts start`) してみたが、以下エラー。

```
$ yarn start
...
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:

"babel-loader": "8.1.0"

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

/Users/user/node_modules/babel-loader (version: 8.0.6)

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:

Delete package-lock.json (not package.json!) and/or yarn.lock in your project folder.
Delete node_modules in your project folder.
Remove "babel-loader" from dependencies and/or devDependencies in the package.json file in your project folder.
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:

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.

Check if /Users/user/node_modules/babel-loader is outside your project directory.
For example, you might have accidentally installed something in your home folder.

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

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

babel-loader@8.1.0 自体はlocal プロジェクトdir にもあり
インストールされている。

解決手順もエラーメッセージにあるが、
試しても上手くいかず。


# 原因

ようは、以前に `brew install node` した際に
`/Users/<user>/node_modules`
にpackage ファイルを置くように NODE_PATH が設定されるようだが、
その時にインストールしたもの が残っていた。

そして、
今回yarn で読み込み時に、別のnode_modules pathを優先的に参照しているようのため
それを直せばいい

# 解決策

とりあえず、参照していた (エラーメッセージにもある通り)
`/Users/<user>/node_modules`
のnode_modulesディレクトリ自体を移動して、参照しないようにした。(安全確認の後、削除)


``` :bash
$ rm -rf /Users/user/node_modules # 対処例として
$ yarn start
yarn run v1.16.0
react-scripts start
...
Compiled successfully!

```

`yarn start` 通った。

MacOS はHomebrew, NVM, Nodebrew など バージョン管理ツールがあるが、
環境変数 Path などが違うようなので注意。


# 参考

https://github.com/facebook/create-react-app/issues/8781

https://stackoverflow.com/questions/59633499/why-cant-i-run-yarn-start