【React1】创建单页应用

Create React App 脚手架工具，是创建React项目的最佳方式。

创建项目前要满足 Node >= 8.10 和 npm >= 5.6。
如何安装和使用React脚手架

npm install -g create-react-app
cd Desktop

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

npm install -g create-react-app 安装脚手架
重启项目时运行 npm run start也可以是 npm start
项目名称，必须为小写字母 my-app

项目创建成功

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报错信息如下：

$ 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”: “4.19.1”

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/apple/node_modules/webpack (version: 4.28.2)

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/apple/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!

error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

解决问题思路：

创建一个 .env 文件 SKIP_PREFLIGHT_CHECK=true 添加这句代码，在终端输入npm start重启就好了。

下图参考图：

2019010119564687.png