There is no zelda.prototype. It should have been: Game.prototype.heartIt = function() { console.log( "I heart " + this.title ); }; See: http://jsfid

There is no zelda.prototype. It should have been: Game.prototype.heartIt = function() { console.log( "I heart " + this.title ); }; See: http://jsfiddle.net/a9asJ/1/ http://jsfiddle.net/a9asJ/1/