JavaScript quick and dirty constants

constants.js

(function() {
  var constants = {}; //or window?

  window.const = function() { //or possibly window._ with arg[0] being string = syntax or object literal as well

    if (!args.length) return constants;

    var value = constants[args[0]];

    if (args.length === 1) return value;
      
    if (typeof(value) === 'undefined') constants[args[0]] = args[1], return args[1];
 
    throw "Constant already defined.";
  }
})();

usage.js

//Constant function - see above
var ALPHA = const("ALPHA", "A");

const("ALPHA = 'A'");
const({ALPHA: 'A'})

//Constant function with argument overloading and use of _ instead
_('ALPHA = "A"')
_({ALPHA: 'A'})
_.ALPHA

//Just use the window namespace but then nothing to stop you changing ALPHA ='B'
const('ALPHA', 'A');
const('ALPHA = "A"');
const({ALPHA: 'A'});

ALPHA
window.ALPHA

Quick and very dirty, hard to do it in a satisfactory way
All we want to do is define a value that is immutable
If we take the function route then may only start with $, _, or \unicode escape