Calling helper function in node.js within callback?

Question

I'm fairly new to programming with node.js and am not quite sure why I am getting this error. The function looks to be set up correctly, and I don't believe I have any asynchronous problems b/c those should be account with the self variable I put in place (I think). I did try w/o that too, using simple var consolePrint(...) Anyways, this is my code below and the error log below that.

/* global __dirname */

var express = require('express');
var app = express();
var bodyParser = require('body-parser');

var self = this;

//CALLING HELPER FUNCTION HERE
var server = app.listen(8000, self.consolePrint(server));

app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());
app.use('/public', express.static(__dirname + '/public'));

app.get('/', function (req, res) {
  res.sendFile(__dirname + '/public/views/index.html');
});


//---------------helper function(s)-------------------//
self.consolePrint = function(serverVar){
  var host = serverVar.address().address;
  var port = serverVar.address().port;
  console.log('Example app listening at http://%s:%s', host, port);
}

and error:

C:\Users\Daniel\Desktop\workspace\alarm_clock\index.js:17
var server = app.listen(8000, self.consolePrint(server));
                                   ^
TypeError: undefined is not a function
    at Object.<anonymous> (C:\Users\Daniel\Desktop\workspace\alarm_clock\index.js:17:36)
    at Module._compile (module.js:460:26)
    at Object.Module._extensions..js (module.js:478:10)
    at Module.load (module.js:355:32)
    at Function.Module._load (module.js:310:12)
    at Function.Module.runMain (module.js:501:10)
    at startup (node.js:129:16)
    at node.js:814:3
12 May 01:01:36 - [nodemon] app crashed - waiting for file changes before starting...

Answer 1

这样可以解决问题：

var server = app.listen(8000, function(){self.consolePrint(server)});

Answer 2

You are using the function before defining it. Put the listen function below the 'self.consolePrint' assignment statement or assign the before using it, it will work.