function Person(){}
Person.prototype.dance = function(){};

function Ninja(){}

// Achieve similar, but non-inheritable, results
Ninja.prototype = Person.prototype;
Ninja.prototype = { dance: Person.prototype.dance };

console.log( (new Ninja()) instanceof Person);

// Only this maintains the prototype chain
Ninja.prototype = new Person();

var ninja = new Ninja();

console.log( ninja instanceof Person );