clarify code output

Question

WHY is the output 9?
var x = 0;
for (; x <= 8; x++) {
x += x - x++;
}
console.log(x);

There is something missing me in this line "x += x - x++;" Could someone unwrap this so that I can understand what is happening, please?
Because I thought it would be equivalent to this: "x = x + x - x; x++; " but it's not true!!

Answer 1

x += expression is equivalent to:
x = x + (expression) //notice the parenthesis.
The same is true for other shorthand assignments.
x = x + (x - x++);
x - x++ is always 0 so you are just saying x = x + 0(doing nothing)
when x = 9 the for loop condition is false and we jump to the console.log part.

Answer 2

x++ returns x first and increments the value of x later.
Expressions are evaluated from left to right and when we increment x in this code we are not getting its value anymore we are simply evaluating a math expression and assigning its result to x.
Let's say x = 5 and let's make substitutions:
x = 5 + (5 - 5); //now x = 6 because we increment it but we don't need that 6;
x = 5 + 0 //x is still 6;
x = 5 //x=5 because we are not using the incremented value of x but the returned value of the math expression.

Answer 3

@Kevin Star thank you, but one more thing please:
why x - x++ == 0 ? and not 1 ?