>>I'd insert "in a loop" after strings, as they did some optimizatons for concatenating in a line and probably in directly following lines.
>
>As to 'and probably in directly following lines' I don't think so

I've seen lines like

var stringres;
stringres = str1 + str2 + str3;

being described as special case where string concatenation is optimised.
I'd expect

var stringres;
stringres = "";
stringres = stringres + str1;
stringres = stringres + str2;
stringres = stringres + str3;

to be rewritten to form 1 by an optimizing compiler as such code can be found - if they can inline whole functions, inlining concats should be easy