Question

Today I saw two different types of Javascript function declarations and I’d like to have a deeper understanding of the two:

function Car( model, year, miles ){
   this.model = model;
   this.year    = year;
   this.miles  = miles;
}

/*
 Note here that we are using Object.prototype.newMethod rather than 
 Object.prototype so as to avoid redefining the prototype object
*/
Car.prototype.toString = function(){
    return this.model + " has done " + this.miles + " miles";
};

var civic = new Car( "Honda Civic", 2009, 20000);
var mondeo = new Car( "Ford Mondeo", 2010, 5000);

console.log(civic.toString());

and type 2:

function Car( model, year, miles ){
   this.model = model;
   this.year    = year;
   this.miles  = miles;
   this.toString = function(){
       return this.model + " has done " + this.miles + " miles";
   };
}


var civic = new Car( "Honda Civic", 2009, 20000);
var mondeo = new Car( "Ford Mondeo", 2010, 5000);

console.log(civic.toString());

Specifically the ‘prototype’ and the ‘this.toString’.

Can anyone impart some pearls of JS wisdom?

Answer 1

The primary difference here is that in method 2 you are redefining the method with every new instance of Car you create, which is technically less performant.

One nice thing method 2 does afford you however is that you can create truly private instance variables, like so:

function Person( _age ){
    var age = _age;
    this.canDrink = function(){
        return age >= 21;
    }
}

var p = new Person(25);
p.canDrink() // true
p.age // undefined, because age is not exposed directly

Another advantage to method 1 (besides performance) is that you can now change the functionality of all instances of on object. For example:

function Person( _age ){
    this.age = _age;
}
Person.prototype.canDrink = function(){
    return this.age >= 21;
}

var a = new Person(15),
    b = new Person(25);
a.canDrink() // false
b.canDrink() // true

Person.prototype.canDrink = function(){ return true }
a.canDrink() // true
b.canDrink() // true

this would not be possible with method 2 (without changing it for every instance). Age however, is now exposed:

a.age // 15
b.age // 25