var oldtime = new Date().getTime();
var str = [];
for(i=0;i<100000;i++)
{
str.push('abcabcabcabcabcabcabcabcabcabcabcabc');
}
var newtime = new Date().getTime();
var joinTime = (newtime - oldtime);

var oldtime = new Date().getTime();
var str='';
for(i=0;i<100000;i++)
{
str += 'abcabcabcabcabcabcabcabcabcabcabcabc';
}
var newtime = new Date().getTime();
var concatTime = (newtime - oldtime);

alert("join / concat: " + (joinTime * 1.0 / concatTime));