Javascript Array random(num, enableDuplicate)

Array.prototype.random = function(num, enableDuplicate){
 var array = [];// w w  w.j  ava2s .  com
 var duplicate;
 if(typeof duplicate === "undefined"){
  duplicate = false;
 }
 else{
  duplicate = enableDuplicate;
 }
 if(num <= this.length){
  var index;
  for(var i=0; i<num; i++){
   index = Math.floor(Math.random()*this.length);
   array.push(this[index]); 
   if(!enableDuplicate){
    this.splice(index, 1);
   } 
  }
  return array;
 } 
}
module.exports = Array;