app.js

var sys=require('sys');
var mamlib=require('./mammal');

console.log(sys.inspect(mamlib));
=> {}
WHY???

mammal.js

function Mammal(name){ 
	this.name=name;
	this.offspring=[];
} 
Mammal.prototype.haveABaby=function(){ 
	var newBaby=new Mammal("Baby "+this.name);
	this.offspring.push(newBaby);
	return newBaby;
} 
Mammal.prototype.toString=function(){ 
	return '[Mammal "'+this.name+'"]';
}