Convert a number within a range to a different range. For instance, 3 on a 1-10 scale to a 1-100 scale. Handy for pegging something like alpha (0-1 range) to an x coordinate (0-width of space)

convertRange

function convertRange(OldMin, OldMax, OldValue, NewMin, NewMax){
  var OldRange = OldMax - OldMin;
  var NewRange = NewMax - NewMin;
  return (((OldValue - OldMin) * NewRange) / OldRange) + NewMin;
}

newOpacity = convertRange(0, 10, 3, 0, 100)
console.log (newOpacity)