How do I change variable value using old value of said variable

Question

I have to write scripts that calculate the power of a number. I tried something like that

power=$1
for((i=$1;i>=1;i--));do
power= expr $i * $power
echo $power
done

I basically want this line

power= expr $i * $power

to do this if it were in c++ or java

power = i * power

How to write this?