React：npm start実行時に「However, a different version of webpack-dev-server was detected higher up in the tree:」が発生した時の対処法

Reactのチュートリアルを始めて、ローカルで環境を整えようとした段階で早速エラーで詰まったので、解決法をここに書いておきます

ja.reactjs.org

環境

• Mac OS Mojave version 10.14.6
• node v12.18.2
• npm 6.14.5
• React 16.13.1

エラー全文

npm start時に以下のエラーが発生

> my-app@0.1.0 start /Users/xxxxxxxxxxxx/react-tutorial/my-app
> react-scripts 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:

"webpack": "4.42.0"

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/xxxxxxxxxxxx/node_modules/webpack (version: 4.44.1)

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/xxxxxxxxxxxx/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! my-app@0.1.0 start: `react-scripts start`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the my-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:
npm ERR! /Users/xxxxxxxxxxxx/.npm/_logs/2020-08-04T06_49_24_909Z-debug.log

 

原因 

プロジェクトフォルダ内とは別のところ(今回はホームディレクトリ)にnode_modulesがあって、そっちを参照(?)していたのが原因

解決策

画像のディレクトリ構成通りに.envファイルを作成

.envファイルにSKIP_PREFLIGHT_CHECK=tureと記述し、再度実行することで起動可能

 SKIP_PREFLIGHT_CHECK=ture

 

参考文献

mebee.info

この記事ではホームディレクトリのnode_modulesを消してたけど、俺にそこまでの勇気はなかった