JS: if var x = 5; var y = --x+x++; alert (y), What will be output & how?

Question

if
--x = 4
x++ = 5
then the output is 8 not 9.
Why??

Answer 1

Right, but this case is different. In the statement of y, the --x part has already reduced the value of x to 4. So x is not 5 anymore. Since x has become 4, the later part of y statement will treat x as 4. Therefore, if x is 4, x++ is also 4.

Answer 2

x++ should be 4 since it's post increment.
--x makes x=4 and --x=4
x++ makes x=5 and x++ = 4
So, y = --x+x++ = 4+4 = 8

Answer 3

Thanks @ SY

Answer 4

but if we do it separately then
var x = 5;
alert (x++)
we get 5.