overriding prototype property or function

function Ninja(){
  this.swingSword = function(){
    return true;
  };
}

// Should return false, but will be overridden
Ninja.prototype.swingSword = function(){
  return false;
};

var ninja = new Ninja();
log( ninja.swingSword(), "Calling the instance method, not the prototype method." );

now log showing me true. which means swingSword that were defined in Ninja.prototype has overridden so how can i override the constructor function or property.?? i know that preference is given to constructor variable then why need to define a function or property inside prototype??

Answer

The reason to define a function on the prototype is so that it is shared between all instances. This will save you some memory rather than each instance having its own copy of a function defined in the constructor.

Some other references you might be interested in:

Javascript when to use prototypes

http://javascript.crockford.com/inheritance.html