Question about best practice with JS objects and internal method calls

example.js

// Capitalize the name of the constructor function
var MessageOfTheDay = function (appName) {
  this._readHttpCallback = this._readHttpCallback.bind(this);
	this.appName = appName;
	if (this.lastMessageText === undefined) {
		this.lastMessageText = "";
		}
	};

messageOfTheDay.prototype = {
  sayHello: function () {
		console.log ("hello");
		},

	checkForUpdate: function () {
		var urlTextFile = "http://1999.io/testing/motd/" + this.appName + ".txt";
		var motd = this;

		readHttpFile (urlTextFile, this._readHttpCallback);
		},

  _readHttpCallback: function (s) {
    if (s !== this.lastMessageText) {
      console.log('messageOfTheDay.checkForUpdate: s ==', s);
      this.sayHello();
      }
    }
};

// construct a new instance of the 'class' you just defined
var messageOfTheDay = new MessageOfTheDay(appName);