overriding prototype property or function

Question

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 1

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

Answer 2

This is by design. Do not set the value in the constructor if you want it to return false.

You can also make a setter method:

function Ninja() {
    var swordState = true;
    this.swingSword = function () {
        return swordState;
    };
    this.setSword = function (b) {
        swordState = b;
    };
}

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

var ninja = new Ninja();
console.log(ninja.swingSword(), "Calling the instance method, not the prototype method.");
ninja.setSword(false);
console.log(ninja.swingSword()); // returns false