zoo.js

var haustier = function( name ) {
    this.name = name;
};
haustier.prototype.toString = function() {
    return this.name;
};

var hund = function( name ) {
    this.name = name;
};
hund.prototype = haustier.prototype;

var bello = new hund( 'bello' );
console.log( 'hallo ' + bello );