snippet.js

// run this first on the console

var script= document.createElement('script');
script.src= 'https://raw.github.com/documentcloud/underscore/master/underscore-min.js';
document.head.appendChild(script);

// then run this

var someObj = {
  
  config : {
    
    groupIds : []
    
  },
  
  _generateGroupId : function randGen(){
  
    var newId = Math.floor(Math.random() * 1000);    
    if( _(this.config.groupIds).include(newId) ){
      newId = randGen.apply( this );
      return newId;
    } else {
      this.config.groupIds.push( newId );
      return newId;
    }
  
  }

};

var myArr = Array(1000).join('foo')
                       .split('foo')
                       .map(function(v, i){
                         return i;
                       }).sort(function(){
                         return Math.random() - 0.5;
                       });
                       
myVal = myArr.pop();
myOtherVal = someObj._generateGroupId();