What does the construct x = x || y mean?

It means the title argument is optional. So if you call the method with no arguments it will use a default value of "Error".

It’s shorthand for writing:

if (!title) {
  title = "Error";
}

This kind of shorthand trick with boolean expressions is common in Perl too. With the expression:

a OR b

it evaluates to true if either a or b is true. So if a is true you don’t need to check b at all. This is called short-circuit boolean evaluation so:

var title = title || "Error";

basically checks if title evaluates to false. If it does, it “returns” "Error", otherwise it returns title.