It doesn't really have to add newlines, just something readable.
Anything better than this?
str = "line 1" +
"line 2" +
"line 3";
Almost identical to NickFitz's answer:
var str = [""
,"line 1"
,"line 2"
,"line 3"
].join("");
// str will contain "line1line2line3"
The difference, the code is slightly more maintainable because the lines can be re-ordered without regard to where the commas are. No syntax errors.