javascript object declaration using prototype(including case) -

possible duplicate:
use of 'prototype' vs. 'this' in javascript?

i donot know why developer use javascript prototype object exactly. so, made sample code below. 1 using prototype object, other plain syntax. difference btwn 2 of them? there benefit use prototype syntax, first case?

car.prototype.engine_ = 'bmw'; car.prototype.getengine = function(){   return this.engine_; } car.prototype.setengine = function(engine){   this.engine_ = engine; } function car(){  //....  }      var mycar = new car();     mycar.getengine();     mycar.setengine('benz');     console.debug(mycar.getengine()); 

vs

function car(){   this.engine_ = 'bmw';   this.setengine = function(engine){     engine_ = engine;   }   this.getengine = function() {     return engine_;   }    //...  }  var mycar = new car(); mycar.getengine(); mycar.setengine('benz'); console.debug(mycar.getengine()); 

yes, there difference. using prototype, property or method declared once in object , same instances. using direct properties/methods, these properties , method added every instance of object. here's car constructor:

function car(){this.engine = 'no known engine'}; car.prototype.setengine = function(val){   this.engine = val; } 

now method setengine equal instances of car, engine property can vary per instance. prototypal methods defined once , looked via prototype chain (see this). in case setengine sets 'engine' property of the current instance of car. can do:

var bmw = new car, merc = new car; bmw.setengine('bmw'); merc.setengine('mercedes'); console.log(bmw.engine); //=> 'bmw' console.log(merc.engine); //= 'mercedes'