This is what I came up with in the end. Not sure if this is helpful to anyone else. It converts any value between 0 and 1 to 1. But if the input value is equal to 0 or the input value is equal to 1, the function will return 0 as well.

Resulting in the loopback / switch behavior. It returns 1 only whilst a value is progressing. Hope this makes sense and is correct. Happy to hear your thoughts.

Also happy to hear some further thoughts on if such a function is more performant than an if-statement.

In the end I had to solve this to prove to myself that it's possible to do with math. (of course it is):)

const loopback = (i, m = 100) => {
    //returns 1 while value is progressing, returns 0 if value is 0 or 1
    //based on triangle wave function: https://discourse.processing.org/t/sawtooth-wave-algorithm/27433
    i = clamp(i) //normalize value to 0-1
    return Math.ceil(Math.abs((i*(m)) % (m)) / m)
}


const clamp = (number, min = 0, max = 1) => {
    return Math.max(min, Math.min(number, max));
}

//in my usecase, this is an alternative to following condition:
/*if(i >= 1 || i <= 0) {
  return 0;
} else {
  return 1;
}