Question

here is the fiddle: http://jsfiddle.net/Xh4GU/1
or the code:

function Vector()
{
    var v = new Array(123, 456, 789);

    this.getV = function()
    {
        return v;
    }
}

function Formulas()
{
    this.add = function(x, axis, units)
    {
        x[axis] += units;
    }
}

var vector = new Vector();
var formulas = new Formulas();

var v = vector.getV();
var vAdded = formulas.add(v, 0, 77)

document.write(v);

spits out: 200,456,789

Why is the first index of v being changed?
Thanks

Answer 1

Because the array is passed to formulas.add by reference the argument being passed formulas.add is a reference to the private array v, any changes you make to its contents will remain visible in the future.

The fact that v is private doesn’t protect its contents when you hand out references to v to external code. It does prevent external code from grabbing v for itself and swapping v with another array, but the array itself can be modified (its values changed).