Question

I am trying to figure out how I can add methods to a constructor after I have created it.
In my code below, I cannot use Person’s prototype property to add a new public method which has access to Person’s vars. (Do the functions attached to the prototype property not close over the vars in the main function).
Unlike the first way, the second way works – Person 2. seems like these are called privileged methods –http://www.crockford.com/javascript/private.html.

function Person(name, age){}
Person.prototype.details = function(){ 
    return "name: "+name+", age: "+age;
};

function Person2(name, age){
 this.details = function(){ 
    return "name: "+name+", age: "+age;};
}

var per1 = new Person("jim", 22);
var per2 = new Person2("jack", 28);

per1.details();
//=> ReferenceError: age is not defined
per2.details();
//=> "name: jack, age: 28"

Answer 1

Of course not, the function was declared in a scope, different to the scope where the arguments/variables were declared, so JS wouldn’t know which variables you’re on about. Suppose you had a second closure, or better (well, worse actually) yet: a global variable called name. Which one would JS pick?

Here’s an example for you:

function MyObject(name)
{
    var localVar = 'foobar';
    this.evilMethod = (function(localVar)
    {
        return function()
        {
            console.log('localVar = '+localVar);//=== name
        };
    })(name);
    this.badMethod = function()
    {
        console.log('localVar = '+ localVar);// === 'foobar'
    };
}
var name = 'Global Name';
var anotherClosure = (function(name)
{
    var localVar = name.toLowerCase();
    return function()
    {
        console.log(name);
        console.log(localVar);
    }
})('Bobby');
MyObject.prototype.closureVars = function()
{
    console.log(name);//Global name
    console.log(localVar);//undefined
};

Now first off: this is terrible code, but you get the point: you can have hundreds of variables with the same name, which one JS has to use, might not always be clear.

Giving prototypes access to instance closure variables has other implications, too: you could, for instance change their values, which defeats the point of having a closure in the first place.
But the biggest problem by a country mile would be: multiple instances! If you create a constructor, odds are you’re going to instantiate more than 1 object with it. How would that work, if they all share the same prototype?

Just assign the arguments/variables you want to access in the prototype to a property, like FishBasketGordo’s example does