Pointers in JavaScript?

Can we pass a reference of a variable that is immutable as argument in a function?

Example:

var x = 0;
function a(x)
{
    x++;
}
a(x);
alert(x); //Here I want to have 1 instead of 0

Solution 1:

Since JavaScript does not support passing parameters by reference, you'll need to make the variable an object instead:

var x = {Value: 0};

function a(obj)
{
    obj.Value++;
}

a(x);
document.write(x.Value); //Here i want to have 1 instead of 0

In this case, x is a reference to an object. When x is passed to the function a, that reference is copied over to obj. Thus, obj and x refer to the same thing in memory. Changing the Value property of obj affects the Value property of x.

Javascript will always pass function parameters by value. That's simply a specification of the language. You could create x in a scope local to both functions, and not pass the variable at all.

Solution 2:

This question may help: How to pass variable by reference in javascript? Read data from ActiveX function which returns more than one value

To summarise, Javascript primitive types are always passed by value, whereas the values inside objects are passed by reference (thanks to commenters for pointing out my oversight). So to get round this, you have to put your integer inside an object:

var myobj = {x:0};

function a(obj)
{
    obj.x++;
}

a(myobj);
alert(myobj.x); // returns 1

  

Solution 3:

I have found a slightly different way implement pointers that is perhaps more general and easier to understand from a C perspective (and thus fits more into the format of the users example).

In JavaScript, like in C, array variables are actually just pointers to the array, so you can use an array as exactly the same as declaring a pointer. This way, all pointers in your code can be used the same way, despite what you named the variable in the original object.

It also allows one to use two different notations referring to the address of the pointer and what is at the address of the pointer.

Here is an example (I use the underscore to denote a pointer):

var _x = [ 10 ];

function foo(_a){
    _a[0] += 10;
}

foo(_x);

console.log(_x[0]);

Yields

output: 20

Solution 4:

You refer to 'x' from window object

var x = 0;

function a(key, ref) {
    ref = ref || window;  // object reference - default window
    ref[key]++;
}

a('x');                   // string
alert(x);