— | NN 2 IE 3 ECMA 1
The decrement operator (a unary operator) subtracts 1 from the current value of a variable expression. You can place the operator in front of or behind the variable for a different effect. When the operator is in front of the variable, the variable is decremented before it is evaluated in the current statement. For example, in the following sequence:
one is subtracted from a before being assigned to b. Therefore, both b and a are 4 when these statements finish running. In contrast, in the following sequence:
the subtraction occurs after a is assigned to b. When the statements complete, b is 5 and a is 4.
This behavior impacts the way for-loop-counting variables are defined and used. Typically, a loop counter that counts backwards from a maximum value decrements the counter after the statements in the loop have run. Thus most loop counters place the operator after the counter variable:
Example

    --n
    n--