Johnny wants the red car — fire-engine-red if he can find it — instead of the blue car.Isn’t proper usage without spaces around the mdash?
Johnny wants the red car—fire-engine-red if he can find it—instead of the blue car.
There’s some debate about the spacing around the em dash (or mdash). I’ve seen it both ways. My copy-editor likes it with spaces, but I’m somewhat convinced she’s insane.