I've always stayed away from multi-line strings, but didn't realize until today that they aren't part of the ECMAScript standard:
var str = 'this is a long string, \ that is two lines long.';
It also never dawned on me what exactly the backslash is doing: it is escaping the newline in the source code!
The whitespace at the beginning of each line can't be safely stripped at compile time; whitespace after the slash will result in tricky errors; and while most script engines support this, it is not part of ECMAScript.
Over the years, I've gravitated toward using simple string concatenation when I don't care about preserving newlines:
var str = 'this is a long string, ' + 'that is two lines long.';
and array joining when I do want newlines:
var str = ['this is a long string, ', 'that is two lines long.'].join('\n');
Neither is pretty, but we don't get true multi-line strings until ECMAScript 6:
var str = `this is a long string, that is two lines long.`;