Question

example:

function Foo() {
    this.bla = 1;
    var blabla = 10;
    blablabla = 100;
    this.getBlabla = function () { 
        return blabla; // exposes blabla outside
    }
}

foo = new Foo();

original question:

I know that bla will be assigned to every instance of Foo.
What will happen with blabla?

new question:

what I understand now:

this.bla = 1;     // will become an attribute of every instance of FOO.
var blabla = 10;  // will become a local variable of Foo(**not** an attribute of every    instance of FOO), which could be accessed by any instance of FOO only if there's a method like "this.getBlabla".
blablabla = 100;  // will define a **new** (or change if exist) global(window) variable.

[Question:] Did i understand correctly?

Answer 1

Any internal-methods you give to this — ie: this.method = function () {}; while inside of your Foo constructor function, are all going to have a reference to the blahblah which is unique to each instance of a Foo object.

function Wallet () {
    var balance = 0;
    this.checkBalance = function () { return balance; };
    this.depositAmount = function (amount) { balance += amount; };
}


var wallet = new Wallet();
wallet.checkBalance();   // 0
wallet.depositAmount(3);
wallet.checkBalance();   // 3

But it’s completely protected from access outside of wallet, unless you return it to somebody, from a privileged function.

wallet.balance; // undefined;

(added bit of interest — if balance is a string, a number, or a boolean, even if you return it, that won’t give people editing rights, or even permanent viewing access — scalar variables are passed by value, so you’re just passing the value of balance at the time — if, however, balance was an object, a function or an array, they’d have permanent access to modify the crap out of your internal workings)

Note: methods HAVE to be assigned inside of the constructor for this to work.
Prototypes can’t access internal variables.
Adding methods later won’t give them access to internal variables.

This means that each instance will take up a little more memory, because each has its own copy of the methods, and has its own copy of the vars.
But if what you’re doing requires private data, this would be a good way to get it.