I've read that if I have a for loop, I should not use string concation because it's slow. Such as:

for (i=0;i<10000000;i++) {
    str += 'a';
}

And instead, I should use Array.join(), since it's much faster:

var tmp = [];
for (i=0;i<10000000;i++) {
    tmp.push('a');
}
var str = tmp.join('');

However, I have also read that string concatention is ONLY a problem for Internet Explorer and that browsers such as Safari/Chrome, which use Webkit, actually perform FASTER is using string concatention than Array.join().

I've attempting to find a performance comparison between all major browser of string concatenation vs Array.join() and haven't been able to find one.

As such, what is faster and more efficient JavaScript code? Using string concatenation or Array.join()?