ExpressJS middleware is being called twice

In the following code, middleware logger() is called twice if I put it as a first argument to app.use() but it doesn't get called at all if put as a second argument. Can someone please explain what's happening?

const express = require('express');  const app = express();  const path = require('path');    app.use(express.static(path.join(__dirname,'public')), logger);    const PORT = process.env.PORT || 5000;  app.listen(PORT, () => console.log(`Server started on port ${PORT}`));    function logger(req, res, next) {    console.log("logger, then next");    next();  }  

https://stackoverflow.com/questions/65855889/expressjs-middleware-is-being-called-twice January 23, 2021 at 12:58PM