Javascript: constant properties

Question

Hey,

In javascript, can I declare properties of an object to be constant?

Here is an example object:

   var XU = {
      Cc: Components.classes
   };

or

   function aXU()
   {
      this.Cc = Components.classes;
   }

   var XU = new aXU();

just putting "const" in front of it, doesn't work.

I know, that i could declare a function with the same name (which would be also kind of constant), but I am looking for a simpler and more readable way.

Browser-compatibility is not important. It just has to work on the Mozilla platform, as it is for a Xulrunner project.

Thank you a lot!

Cheers.

Answer 1

UPDATE: This works!

const FIXED_VALUE = 37;
FIXED_VALUE = 43;
alert(FIXED_VALUE);//alerts "37"

Technically I think the answer is no (Until const makes it into the wild). You can provide wrappers and such, but when it all boils down to it, you can redefine/reset the variable value at any time.

The closest I think you'll get is defining a "constant" on a "class".

// Create the class
function TheClass(){
}

// Create the class constant
TheClass.THE_CONSTANT = 42;

// Create a function for TheClass to alert the constant
TheClass.prototype.alertConstant = function(){
  // You can’t access it using this.THE_CONSTANT;
  alert(TheClass.THE_CONSTANT);
}

// Alert the class constant from outside
alert(TheClass.THE_CONSTANT);

// Alert the class constant from inside
var theObject = new TheClass();
theObject.alertConstant();

However, the "class" TheClass itself can be redefined later on

Answer 2

If you are using Javascript 1.5 (in XUL for example), you can use the const keyword instead of var to declare a constant.

The problem is that it cannot be a property of an object. You can try to limit its scope by namespacing it inside a function.

(function(){ 

const XUL_CC = Components.classes;

// Use the constant here

})()

Answer 3

Since you only need it to work on the Mozilla platform, you can define a getter with no corresponding setter. The best way to do it is different for each of your examples.

In an object literal, there is a special syntax for it:

var XU = {
    get Cc() { return Components.classes; }
};

In your second exampe, you can use the __defineGetter__ method to add it to either aXU.prototype or to this inside the constructor. Which way is better depends on whether the value is different for each instance of the object.

Edit: To help with the readability problem, you could write a function like defineConstant to hide the uglyness.

function defineConstant(obj, name, value) {
    obj.__defineGetter__(name, function() { return value; });
}

Also, if you want to throw an error if you try to assign to it, you can define a setter that just throws an Error object:

function defineConstant(obj, name, value) {
    obj.__defineGetter__(name, function() { return value; });
    obj.__defineSetter__(name, function() {
        throw new Error(name + " is a constant");
    });
}

If all the instances have the same value:

function aXU() {
}

defineConstant(aXU.prototype, "Cc", Components.classes);

or, if the value depends on the object:

function aXU() {
    // Cc_value could be different for each instance
    var Cc_value = return Components.classes;

    defineConstant(this, "Cc", Cc_value);
}

For more details, you can read the Mozilla Developer Center documentation.