test.js

(function() {
     var stuff = "str";

     // creates a really long string
     var str = "";
     for (var i = 0; i < 10000; ++i) {
         str += 'stuff + ';
     } 

     eval("stuff = 'str'; var d = +new Date; x = " + str
         + " stuff; console.log('Concat: ' "
         +"+ (+new Date - d));");
     

     // creates a really long array
     var arr = [];
     var d = +new Date;
     for (var i = 0; i <= 10000; ++i) {
         arr[i] = stuff;
     }
     arr.join("");
     console.log('Join: ' + (+new Date - d));
}());