javascript how to create reference

Question

Could you propose any workarounds to implement a reference to variable using closures or any other tricks?

createReference = function() {
    // TODO: how to implement?
};

var x = 5;
var refX = createReference(x); // could be any parameters needed to implement the logic
x = 6;
alert(refX()); // should alert 6

What about passing context as first argument and pass variable name (as string) and later somehow evaluate that reference in predefined context. Is this feasible?

Here's a more complete scenario:

createReference = function(context, prop) {
    return function() {
        return context[prop];
    };
};

Provider = function() {
};
Provider.prototype.x = 5;
Provider.prototype.getXRef = function() {
    return createReference(this, 'x');
};
Provider.prototype.incrementX = function() {
    this.x = this.x + 1;
};

var provider = new Provider();
var refX = provider.getXRef();
provider.incrementX();
alert(refX());

Answer 1

You have to use a string of the variable name but I think this is as close as you'll ever get in JavaScript:

var createReference = function (context, prop) {
  return function () { return context[prop]; };
};

var x = 5;
var refX = createReference(this, 'x');
x = 6;

alert(refX()); // alerts 6

Edit:

In your updated scenario it would be better to use a closure directly, so that you don't have to use a string of the variable name:

var createReference = function (context, func) {
    return function () { return func.call(context); }
};

Provider = function() {
};
Provider.prototype.x = 5;
Provider.prototype.getXRef = function() {

    return createReference(this, function () { return this.x; });

    // OR if you happen to be running in a 
    // JavaScript 1.8 environment like Firefox 3+,
    // you can use "expression closures" for more
    // concise code:

    // return createReference(this, function () this.x);
};
Provider.prototype.incrementX = function() {
    this.x = this.x + 1;
};

var provider = new Provider();
var refX = provider.getXRef();
provider.incrementX();
alert(refX()); // alerts 6

Answer 2

In JavaScript, you can't pass primitive values (numbers, strings, etc) by reference. However, every object you pass will always be by reference. (this includes arrays)

To use your example:

var foo = { x: 5 };
var refFoo = foo;

// foo.x => 5
// refFoo.x => 5

foo.x = 6;

// foo.x => 6
// refFoo.x => 6