mocking constant behaviour using `Object.defineProperty`, works on real browsers & IE9+

constant.js

/**
 * simulating constant behaviour where
 * given value cannot be changed
 *
 * @param {object} target object where the constant will be defined
 * @param {string} key
 * @param {object} value
 * @return {object} constant
 * 
 * example
 * -------
 * Constant( 'cantTouchThis', 99 );
 * cantTouchThis; // 99
 * cantTouchThis = 200; // 99
 */

function Constant( key, value, target ) {
  
  target = target || window;

  if( value == null ) {
    throw new Error( 'constant should have a value at the time of creation' );
  }

  Object.defineProperty( target, key, { value : value, enumerable: true });

  return target[ key ];
}