React.js npm start時にエラー「However, a different version of webpack-dev-server was detected higher up in the tree:」が発生した場合の対処法

React.js npm start時にエラー「However, a different version of webpack-dev-server was detected higher up in the tree:」が発生した場合の対処法

create-react-appで作成したプロジェクトが、突然エラー「However, a different version of webpack-dev-server was detected higher up in the tree:」が発生して起動できくなった時の、原因と対処法を記載します。

環境

  • OS  CentOS Linux release 8.0.1905 (Core)
  • node V12.13.1
  • npm 6.14.1
  • React 16.13.0

エラー全文

npm start時に下記のエラーが発生

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-dev-server": "3.10.2"

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

  /home/testuser/node_modules/webpack-dev-server (version: 3.10.1) 

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-dev-server" 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-dev-server is outside your project directory.
     For example, you might have accidentally installed something in your home folder.

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

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:

記述通りに作成したプロジェクト配下に.envファイルを作成し「SKIP_PREFLIGHT_CHECK=true」を記述すれば、エラーはなくなり起動できますが、腑に落ちないので、調査。

If you would prefer to ignore this check, add SKIP_PREFLIGHT_CHECK=true to an .env file in your project.

参考.envを作成した対応

ディレクトリ構成通りに.envファイルを作成

.envに SKIP_PREFLIGHT_CHECK=true と記述し、チェックを無視した状態で、再度実行すると起動はできます。

SKIP_PREFLIGHT_CHECK=true

原因

webpackが作成したディレクトリ以外に、インストールされていたことが原因

対処法

自分の環境ではホームディレクトリにある「 node_modues 」を全て削除で解決しました。

※自分の環境はテスト環境なので削除しても問題なかったが、削除時は、慎重に対応してください。