expression counting from 0% to 100% and start at 0 again.

Hello,

I looked through the forum for an answer to this question, but haven't found one.
So as it is written in the title I am looking for an expression which counts from 0% to 100% and starts at 0% all over to 100% again in a loop.

I just found this expression saying that the counter runs from 0% to 100% but then decreases to 0% and starts counting upwards to 100% again.

freq = 1;
50*(1 - Math.cos(freq*time*Math.PI*2))

so I believe there must be some kind of "if" like "if the number reaches 100 start at 0 again" or something. but I have not much experience so I hope for some advices!