const MIN = 1;
const MAX = 20;

Math.min(Math.max(input, MIN), MAX);

The way this works is, assuming the input is larger than the expected range.

Math.max() will pick the the larger value, then pass it on to Math.min() which then picks MAX as the smaller value.

The concept is reversed when the input is smaller.

I also just learnt that this concept is called clamping.