「npm start」をすると「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:」のエラー文が出て来たときの対処法

#この記事について
初学者向けの記事となっています。
https://react-projects.netlify.app/
こちらのサイトでReactを使って実装する際に、**npm start**をしたところタイトルの通りのエラーが出ました。

タイトルのエラー文を訳すとこんな感じ
**「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。
ただし、ツリーの上位で異なるバージョンのwebpackが検出されました」**
つまり、**「上位のディレクトリにwebpackが重複してインストールされてるぞ」**ってことですかね。

確かに私のアプリディレクトリには「node_module」が入っていたのでそこと重複しているっぽい。
しかし**node_moduleを削除しても変わらなかった為、最終手段に出ました。**

##結論
いきなり結論になりますがアプリディレクトリの直下に「.env」ファイルを作成し下記の一行を加える事で、解決しました。

```.env
SKIP_PREFLIGHT_CHECK=true
```
##エラー文
因みにエラー文の全文はこちら
長くてすみません。。。

```terminal

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": "4.44.2"

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/ishiwatahiroshishou/node_modules/webpack (version: 5.1.3) 

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 "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 /Users/ishiwatahiroshishou/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! reminder@0.1.0 start: `react-scripts start`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the reminder@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:
npm ERR!     /Users/ishiwatahiroshishou/.npm/_logs/2020-11-03T10_08_18_340Z-debug.log
```

このエラー文の中のこちらの一行に注目しました。

```
If nothing else helps, add SKIP_PREFLIGHT_CHECK=true to an .env file in your project.
```
訳すと「他に何も役に立たない場合は、プロジェクトの.envファイルにSKIP_PREFLIGHT_CHECK = trueを追加します。」
つまり最終手段としてこれを使えってことか...

**これを加えてnpm start!!**

```terminal
Compiled with warnings.

src/App.js
  Line 1:17:  'useState' is defined but never used     no-unused-vars
  Line 2:8:   'SingleColor' is defined but never used  no-unused-vars
  Line 4:8:   'Values' is defined but never used       no-unused-vars

src/SingleColor.js
  Line 1:17:  'useState' is defined but never used   no-unused-vars
  Line 1:27:  'useEffect' is defined but never used  no-unused-vars
  Line 2:8:   'rgbToHex' is defined but never used   no-unused-vars

src/utils.js
  Line 3:21:  Expected '===' and instead saw '=='  eqeqeq

Search for the keywords to learn more about each warning.
To ignore, add // eslint-disable-next-line to the line before.
```

起動できました！！
あとはlocalhost:3000にアクセスすれば起動が確認できます。

ただしちょっと腑に落ちないので、今後ちょっと深ぼってみようと思います。

**あくまで最終手段であるという事をお忘れずに！！**