Scale to fit

scaleToFit.js

function scaleToFit (
  contentWidth, 
  contentHeight, 
  containerWidth, 
  containerHeight
) {
  var scale = Math.min(containerWidth / contentWidth, containerHeight / contentHeight)
  return {
    x: containerWidth / 2 - contentWidth * scale / 2,
    y: containerHeight / 2 - contentHeight * scale / 2,
    scale: scale
  }
}