[FMX]d++ : Simple question about variables and incrementing

Well I am working through some tile-based games tutorials and saw this:

this.attachMovie("tile", "t_"+i+"_"+j, ++d);

But it doesn’t seem that d has been declared anywhere. It may be a stupid question, but is it simply declared there as 0? Created when incremented? Seems odd. If that’s the case, I didn’t know you could do that. Any one wanna explain it to me…