我想问一下这个事件是怎样被触发的？

我想问一下面这段代码中的两个server.on是怎样被触发的？（斜体字部分）

******#!/usr/bin/env node/

• Module dependencies. */

var app = require('../app'); var debug = require('debug')('iothub-server:server'); var http = require('http');

/**

• Get port from environment and store in Express. */

var port = normalizePort(process.env.PORT || '3000'); app.set('port', port);

/**

• Create HTTP server. */

var server = http.createServer(app);

/**

• Listen on provided port, on all network interfaces. */

server.listen(port); server.on('error', onError); server.on('listening', onListening);

/**

• Normalize a port into a number, string, or false. */

function normalizePort(val) { var port = parseInt(val, 10);

if (isNaN(port)) { // named pipe return val; }

if (port >= 0) { // port number return port; }

return false; }

/**

• Event listener for HTTP server "error" event. */

function onError(error) { if (error.syscall !== 'listen') { throw error; }

var bind = typeof port === 'string' ? 'Pipe ' + port : 'Port ' + port;

// handle specific listen errors with friendly messages switch (error.code) { case 'EACCES': console.error(bind + ' requires elevated privileges'); process.exit(1); break; case 'EADDRINUSE': console.error(bind + ' is already in use'); process.exit(1); break; default: throw error; } }

/**

• Event listener for HTTP server "listening" event. */

function onListening() { var addr = server.address(); var bind = typeof addr === 'string' ? 'pipe ' + addr : 'port ' + addr.port; debug('Listening on ' + bind); }