[javascript] When should I use double or single quotes in JavaScript?

I use single quotes most of the time, because when developing in PHP, single quoted-string are in no way altered, which is what I want. When I use

echo "$xyz";

In PHP, $xyz gets evaluated, which is not what I want. Therefore I always use ' instead of " when it comes to web development. So I ensure at least string-consistency when it comes to PHP/JavaScript.

Unfortunately this can't be done in Java or Objective-C, where '' stands for character and "" stands for string. But this is another question.