场景：使用create-react-app创建项目，由于前后分离开发，本地起一个服务http://localhost:3000。所以当使用自带fetch去请求API的时候就存在同源策略跨域问题，今天分享一个跨域调试小技巧，无需服务端配合。

方法一：

安装谷歌浏览器跨域插件：Allow-Control-Allow-Origin
注意：fetch请求中配置不可填写credentials: "include"

方法二：

通过proxy解决跨域问题，可以直接在package.json下配置，具体如下：

"proxy": "http://api.xxxx.com"

满足可以调用不同域下的API的需求，代码如下：

//package.json中加入
  "proxy": {
    "/api/RoomApi": {
      "target": "http://api.xxxx.com",
      "changeOrigin":true
    },
    "/api/v1":{
      "target":"http://api.xxxx.com",
      "changeOrigin":true
    }
}

注意：第二种方法fetch在请求的时候要使用相对路径url，即：url='/master/login'。不可使用全路径：即：'http://api.xxxx.com/master/login'

我们在实际开发中，打包后上传到同域名路径就ok了，不需要设置webpack代理的，这时候我们在打包的时候就需要在package.json配置中关闭刚才设置的proxy，挺麻烦的，难免打包的时候忘记屏蔽掉。解决方法：

    
1.你也可以使用 create-react-app 提供的  yarn run eject 命令将所有内建的配置暴露出来
2.修改webpackDevServer.config.js文件添加
```注意：yarn run eject可能导致项目跑不起来，这时候执行yarn 或者 npm install即可 ```

(process.env.NODE_ENV === 'development') && (proxy = {
    "/": {
      "target": "http://api.xxxx.com",
      "changeOrigin": true
    }
  });

webpackDevServer.config.js完整代码：

'use strict';

const errorOverlayMiddleware = require('react-dev-utils/errorOverlayMiddleware');
const noopServiceWorkerMiddleware = require('react-dev-utils/noopServiceWorkerMiddleware');
const ignoredFiles = require('react-dev-utils/ignoredFiles');
const config = require('./webpack.config.dev');
const paths = require('./paths');

const protocol = process.env.HTTPS === 'true' ? 'https' : 'http';
const host = process.env.HOST || '0.0.0.0';
module.exports = function(proxy, allowedHost) {

  (process.env.NODE_ENV === 'development') && (proxy = {
    "/": {
      "target": "https://echo-dev.skyplan.online",
      "changeOrigin": true
    }
  });
  
  return {
    // WebpackDevServer 2.4.3 introduced a security fix that prevents remote
    // websites from potentially accessing local content through DNS rebinding:
    // https://github.com/webpack/webpack-dev-server/issues/887
    // https://medium.com/webpack/webpack-dev-server-middleware-security-issues-1489d950874a
    // However, it made several existing use cases such as development in cloud
    // environment or subdomains in development significantly more complicated:
    // https://github.com/facebookincubator/create-react-app/issues/2271
    // https://github.com/facebookincubator/create-react-app/issues/2233
    // While we're investigating better solutions, for now we will take a
    // compromise. Since our WDS configuration only serves files in the `public`
    // folder we won't consider accessing them a vulnerability. However, if you
    // use the `proxy` feature, it gets more dangerous because it can expose
    // remote code execution vulnerabilities in backends like Django and Rails.
    // So we will disable the host check normally, but enable it if you have
    // specified the `proxy` setting. Finally, we let you override it if you
    // really know what you're doing with a special environment variable.
    disableHostCheck:
      !proxy || process.env.DANGEROUSLY_DISABLE_HOST_CHECK === 'true',
    // Enable gzip compression of generated files.
    compress: true,
    // Silence WebpackDevServer's own logs since they're generally not useful.
    // It will still show compile warnings and errors with this setting.
    clientLogLevel: 'none',
    // By default WebpackDevServer serves physical files from current directory
    // in addition to all the virtual build products that it serves from memory.
    // This is confusing because those files won’t automatically be available in
    // production build folder unless we copy them. However, copying the whole
    // project directory is dangerous because we may expose sensitive files.
    // Instead, we establish a convention that only files in `public` directory
    // get served. Our build script will copy `public` into the `build` folder.
    // In `index.html`, you can get URL of `public` folder with %PUBLIC_URL%:
    // <link rel="shortcut icon" href="%PUBLIC_URL%/favicon.ico">
    // In JavaScript code, you can access it with `process.env.PUBLIC_URL`.
    // Note that we only recommend to use `public` folder as an escape hatch
    // for files like `favicon.ico`, `manifest.json`, and libraries that are
    // for some reason broken when imported through Webpack. If you just want to
    // use an image, put it in `src` and `import` it from JavaScript instead.
    contentBase: paths.appPublic,
    // By default files from `contentBase` will not trigger a page reload.
    watchContentBase: true,
    // Enable hot reloading server. It will provide /sockjs-node/ endpoint
    // for the WebpackDevServer client so it can learn when the files were
    // updated. The WebpackDevServer client is included as an entry point
    // in the Webpack development configuration. Note that only changes
    // to CSS are currently hot reloaded. JS changes will refresh the browser.
    hot: true,
    // It is important to tell WebpackDevServer to use the same "root" path
    // as we specified in the config. In development, we always serve from /.
    publicPath: config.output.publicPath,
    // WebpackDevServer is noisy by default so we emit custom message instead
    // by listening to the compiler events with `compiler.plugin` calls above.
    quiet: true,
    // Reportedly, this avoids CPU overload on some systems.
    // https://github.com/facebookincubator/create-react-app/issues/293
    // src/node_modules is not ignored to support absolute imports
    // https://github.com/facebookincubator/create-react-app/issues/1065
    watchOptions: {
      ignored: ignoredFiles(paths.appSrc),
    },
    // Enable HTTPS if the HTTPS environment variable is set to 'true'
    https: protocol === 'https',
    host: host,
    overlay: false,
    historyApiFallback: {
      // Paths with dots should still use the history fallback.
      // See https://github.com/facebookincubator/create-react-app/issues/387.
      disableDotRule: true,
    },
    public: allowedHost,
    proxy,
    before(app) {
      // This lets us open files from the runtime error overlay.
      app.use(errorOverlayMiddleware());
      // This service worker file is effectively a 'no-op' that will reset any
      // previous service worker registered for the same host:port combination.
      // We do this in development to avoid hitting the production cache if
      // it used the same host and port.
      // https://github.com/facebookincubator/create-react-app/issues/2272#issuecomment-302832432
      app.use(noopServiceWorkerMiddleware());
    },
  };
};