请教webpack和nodejs的跨域问题，ajax总是404。万分感谢啊！

Question

      各位师兄好啊。我在做一个用户名登录功能测试的时候，利用ajax以及webpack服务器post跨域请求express服务器的时候遇到点麻烦。我先输入username，然后onblur之后发送用户名至后台，后台res.send(req.body.username),然后ajax的success把data打印出来看。然而，却总是404，估计是跨域失败的问题？
      webpack代码如下：

var path=require('path');
var webpack=require('webpack');

var ExtractTextPlugin = require("extract-text-webpack-plugin"); 
var CommonsChunkPlugin = require("webpack/lib/optimize/CommonsChunkPlugin");

var proxy=require("http-proxy-middleware");

module.exports={
    devtool: 'eval-source-map',
    entry: {
        index: [
            'webpack-dev-server/client?http://localhost:8181',
            'webpack/hot/only-dev-server',
            './public/javascripts/entry.js'
        ],
        vendor: ['react', 'react-dom', 'react-router', 'jquery']
    },
    output:{
        // path: path.resolve(__dirname,'./build'),
        filename:'bundle.js',
        publicPath: '/'
    },
    devServer: { //这儿有问题？ 重定向失败的问题？
        // contentBase: "./",
        inline: true,
        historyApiFallback: true,
        port:8181,
        host: "127.0.0.1",
        proxy: [
            {    
                context: "/*/*", //跨域的路由设置
                target: "http://localhost:3000",  //跨到哪儿去^-^
                changeOrigin: true  //允许跨域，必不可少
            }
        ]
    },
    module: {
        loaders: [
            {
                test: /\.js?$/,
                exclude: /node_modules/,
                loader: 'babel-loader',
                query: {
                    presets:['es2015', 'stage-0', 'react'],
                    plugins: [
                        ['import', {libraryName: 'antd', style: 'css'}]
                    ]
                }
            },
            {
                test: /\.css$/, 
                loader: ExtractTextPlugin.extract({
                    fallback: 'style-loader',
                    use: 'css-loader'
                })
            },
            {
                test: /\.scss$/, 
                loader: ExtractTextPlugin.extract({
                    fallback: 'style-loader',
                    use: 'css-loader!sass-loader'
                })
            },
            {
                   test: /\.(png|jpg)$/, 
                loader: 'url-loader?limit=8192'
            }
        ]
    },
    plugins: [
        new webpack.HotModuleReplacementPlugin(),
        new ExtractTextPlugin('style.css'),
        new CommonsChunkPlugin({
           name: 'vendor',
           filename: 'vendor.js'
        })
    ]
}

      express服务端代码如下，主要是最下面一点的设置：

var environment=process.env.NODE_ENV;
var webpack=require('webpack');
var config;

var express = require('express');
var path = require('path');
var favicon = require('serve-favicon');
var logger = require('morgan');
var cookieParser = require('cookie-parser');
var bodyParser = require('body-parser');
var consolidate = require('consolidate');
var app = express();

if (environment==='production') {
  config=require('./webpack.config.prod.js');
} else if (environment==='development') {
  config=require('./webpack.config.dev.js');
} else {
  console.log('Please define NODE_ENV first');
}

var compiler=webpack(config);
if (environment==='production') {
  var WebpackDevMiddleware=require('webpack-dev-middleware');
  app.use(WebpackDevMiddleware(compiler, { 
    noInfo: true, 
    publicPath: config.output.publicPath 
  }));
} else if (environment==='development') {
  var WebpackHotMiddleware=require('webpack-hot-middleware');
  app.use(WebpackHotMiddleware(compiler));
}

var index = require('./routes/index');
var users = require('./routes/users');

// view engine setup
// app.engine('html', consolidate.ejs);
app.set('view engine', 'ejs'); 
app.set('views', path.join(__dirname, 'views'));

// uncomment after placing your favicon in /public
//app.use(favicon(path.join(__dirname, 'public', 'favicon.ico')));
app.use(logger('dev'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'views')));

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

// 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');
});

//跨域请求设置
app.all('*', function(req, res, next) {
    res.header('Access-Control-Allow-Origin', '*');
    res.header('Access-Control-Allow-Methods', 'PUT, GET, POST, DELETE, OPTIONS');
    res.header('Access-Control-Allow-Headers', 'X-Requested-With');
    res.header('Access-Control-Allow-Headers', 'Content-Type');
    next();
});

//here my codes
app.post('/check/:item', function(req, res) {
  if (req.params.item=="user") {
    var user=req.body.username;
    res.send(user);
  }
})
app.listen(3000, function(err) {  
  if (err) {
    console.error(err);
  } else {
    console.log("listen on port 3000");
  }
})

      最后是ajax的代码，其他地方都没问题，onchange取到user.value的值然后发送后台：

$.ajax({
            type: "post",
            url: "check/user", //配置过devServer中的proxy  
            // dataType: "json", 
            data: {
                username: user.value
            },
            success: (data)=>{
                console.log(data, $.type(data));return;
            },
            error: (xhr)=>{
                console.log("出错啦："+xhr.status)
            }
        });

尝试修改过的部分包括以下，但是都不行：

1. ajax的url写成 /check/user 或者localhost:服务器端口号
2. webpack的devServer的contentBase处，以及proxy写成：

proxy: {
    "/check/*": {    
                secure: false,
                target: "http://localhost:3000",  //跨到哪儿去^-^
                changeOrigin: true  //允许跨域，必不可少
            }
        }

Answer 1

算了，本来准备关闭问题，还是把问题的出处说一下。
ajax一直出现404的原因，并不是跨域不成功的问题，也不是ajax写法的问题，而是后台app.js里面的404错误捕获造成的：

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

我经过把后台所有代码一段一段屏蔽查看影响的时候，发现屏蔽了以上代码后，ajax就能正常返回data的值，而不是404错误。但是我不太理解为什么会这样？
这个地方难道不是只有在出现404错误的时候才把错误返回到前台吗，为什么并没有错误就返回404？？？

Answer 2

app.post('/check/:item', function(req, res) {
这里你是字符串那就是固定死了，就只会匹配/check/:item,应该用正则匹配