以下代码何时调用/执行/调用JavaScript函数表达式?

var express = require('express')

var app = express();

app.use(express.static('static'));

var server = app.listen(3000, function() {
    var port = server.address().port;
    console.log("Started server at port", port);
});
var express = require('express')

var app = exp