ES5 this Example

gistfile1.js

function Gamer() {
	// 'this' refers to Gamer
	this.playTime = 0;
	
	// We think trackPlayTime() will update a gamer's playtime every second
	setInterval( function trackPlayTime() {
		// But we're wrong because trackPlayTime() creates it's own version of 'this'
		this.playTime++;
		// Outputs NaN, because this.playTime was never cast as an integer in this scope
		console.log( this.playTime ); 
	}, 1000 );
	
	// Outputs 0, because this.playTime was never incremented in this scope
	console.log( this.playTime );
}

var gamer = new Gamer();