Why does this code alert 1 and not 0?

Question

var a = 13;
var b = a--;
alert(b-a)

Answer 1

Because,
in this code,
the value of b becomes 13 not 12 because it is a-- not --a.
and the value of a becomes 12
so b-a=13-12=1
See the last part of this lesson:
https://www.sololearn.com/learn/JavaScript/1130/?ref=app