為什么中間件的內容在瀏覽器的單個請求中運行四次

Question

為什么console.log('First Log')每個請求運行4次？

//app.js
const express = require('express');
var app = express();

app.use((req, res, next) => {
  console.log('First Log'); // problem is here
  next();
});

app.use((req, res, next) => {
  res.send('first response from express');
});

module.exports = app;

//server.js
const http = require('http');
const app  = require('./backend/app');

var port =  process.env.PORT || 3000;

app.set('port', port);

var server = http.createServer(app);
server.listen(port);

輸出：

First Log
First Log
First Log
First Log

Answer 1

中間件可以是所有路徑通用的，也可以僅在服務器處理的特定路徑上觸發。 以下是中間件聲明的示例。

var app = express();
app.use(function () {}) //added to all paths or globally
app.get('/someroute', function() {}) //added to a specific path

參考： https : //medium.com/@agoiabeladeyemi/a-simple-explanation-of-express-middleware-c68ea839f498

@AikonMogwai在評論中提到的答案也是正確的：

The middleware works one time for each url(of the same route path): index.html, favicon.ico, *.css, etc. 
Change console.log to console.log(req.url) to see that.