Chrome/Firefox 在后台发送两个 POST，间隔恰好 5 秒，仅调用一次来获取 Nodejs 8.0.0 服务器

var http = require("http");
// A counter for the requests
var counter = 0;

var server = http.createServer(function(request, response) {
  // Closure to prevent the request number from changing with the counter.
  (function(requestNumber) {
    console.log('Incoming:', request.method, "assigning number", requestNumber);

    // CORS headers
    response.writeHead(200, {
      "Content-Type": "application/json",
      "Access-Control-Allow-Methods": "GET,HEAD,PUT,POST,DELETE",
      "Access-Control-Allow-Origin": "*",
      "Access-Control-Allow-Headers": "authorization,content-type,expect",
    });

    // Respond to options immediately.
    if (request.method === "OPTIONS") {
      response.end()
    } else {
      // Otherwise simulate a long running process (db write whatever)
      setTimeout(function() {
        console.log('Timeout done, responding to request number', requestNumber);
        response.write(JSON.stringify({ number: requestNumber }));
        response.end();
      }, 10000);
    }
  }(counter++));
});

server.listen(8080);
console.log("Server is listening");

var start = new Date();
console.log('making call. Wait ~15 seconds for the result to come back');
fetch('http://localhost:8080', {
  headers: {
    Accept: 'application/json',
    'Content-Type': 'application/json',
  },
  method: 'post',
  mode: 'cors',
  body: JSON.stringify({ some: 'content' }),
}).then(result => result.json()).then(json => {
  console.log('result', json);
  console.log('call took:', (new Date() - start), 'seconds');
}).catch(err => {
  console.log('err', err);
});