Node.js中Express路由变量页面刷新后为何持续保留？

Let's start with your code for context:

const express = require('express'); 
const route = express.Router() ; 
let a = 0; 
route.get('/', (req,res)=> { 
  a++; 
  res.send(`hello api we are here! ${a}`) 
}) 
module.exports = route;

You noticed that the first visit to localhost shows hello api we are here! 1, and every refresh increments the number — but you expected the server to reload from scratch for each browser connection. Here's what you're missing:

• Your Express server runs as a single, long-lived process: Once you start your server (e.g., with node server.js), it stays alive in memory until you manually stop it. It doesn't shut down and restart for every incoming request — that would be terribly inefficient.
• Module-level variables are initialized once: Your let a = 0 is declared at the top level of your route module. When the server starts up, this module is loaded one time, and the a variable is created in the module's scope. Every subsequent request to the / route reuses this same variable — it never resets to 0 unless you restart the server.
• Scope determines variable lifetime: If you wanted a variable that's fresh for every request, you'd declare it inside the route handler function (like moving let a = 0; inside the (req, res) => {...} block). But since you're tracking total requests, keeping a at the module level makes perfect sense — just remember it'll only reset when you restart your server.

In short: Your server isn't reloading for each connection. It's a persistent process that retains your module's variables in memory between requests, which is why a keeps counting up.

内容的提问来源于stack exchange，提问作者aryan