IT技术 - Express 服务器和 Axios CORS 预检响应不成功 - 吾爱随笔录

我使用 react.js 执行以下获取操作（在端口 3000 上运行）

var config = {
  headers: {
    'Access-Control-Allow-Origin': '*',
    'Access-Control-Allow-Headers': 'Origin, X-Requested-With, Content-Type, Accept',
    // 'Accept': 'application/json',
    // 'Content-Type': 'application/json',
  }
};
axios.get(api_url + '/schools/countries/' + country, config)
  .catch(err => {
    alert('There was an error trying to fetch', country)
  })
  .then(response => {
    .....

}

我正在使用代理将其发送到在端口 3001 上运行的快速服务器。快速服务器中的 App.js 文件如下：

var express = require('express');
var app = express();
var cors = require('cors')
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');

// uncomment after placing your favicon in /public
//app.use(favicon(path.join(__dirname, 'public', 'favicon.ico')));
//app.use(logger('dev'));

// app.use(function(req, res, next) {
//   res.header("Access-Control-Allow-Origin", "*");
//   res.header("Access-Control-Allow-Methods", "GET,HEAD,OPTIONS,POST,PUT");
//   res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept, Authorization");
//   next();
// });

app.all('*', function(req, res, next) {
  res.header('Access-Control-Allow-Origin', 'URLs to trust of allow');
  res.header('Access-Control-Allow-Methods', 'GET, POST, OPTIONS, PUT, PATCH, DELETE');
  res.header('Access-Control-Allow-Headers', 'Content-Type');
  if ('OPTIONS' == req.method) {
    res.sendStatus(200);
  } else {
    next();
  }
});


//app.use(cors())
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({
  extended: false
}));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));

app.use('/', index);
app.use('/users', users);
app.use('/schools', schools);

console.log("IN APP.JS");
// catch 404 and forward to error handler
app.use(function(req, res, next) {
  var err = new Error('Not Found');
  err.status = 404;
  next(err);
});

// error handler
app.use(function(err, req, res, next) {
  // set locals, only providing error in development
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};

  // render the error page
  res.status(err.status || 500);
  res.render('error');
});

module.exports = app;

出于某种原因，我收到以下错误：

[错误] 加载资源失败：预检响应不成功（MR，第 0 行）

[错误] XMLHttpRequest 无法加载http://localhost:3000/schools/countries/MR。预检响应不成功

在react端（前端）我有这个 package.json 文件：

"proxy": "http://localhost:3001",

我试过在 App.js 文件中使用不同的头文件，我试过使用 cors 库来完成这项工作等。我该如何解决这个错误？

谢谢！