Stuck in augmented substraction

Can you help me please when I add addition in subtract augmentation it is not working properly. My code is

{ var subtract=10
subtract-= 1 + 1; }

I mean 10-1+1 which is equal to 10. But when I run my console it shows 9.

Please help me.

Well, it should be 8.
According to operator precedence unary plus (+) has higher precedence than assignment (-=), therefore summation (1 + 1) is done first and then assignment (subtract -= 2), which result in 8.

Thanks a lot for your help.