like this
var number = Math.min(Math.max(parseInt(number), 1), 20);
Live Demo:
function limitNumberWithinRange(num, min, max){
const MIN = min || 1;
const MAX = max || 20;
const parsed = parseInt(num)
return Math.min(Math.max(parsed, MIN), MAX)
}
alert(
limitNumberWithinRange( prompt("enter a number") )
)