Contain one aspect ratio within another aspect ratio

contain.js

/* Simulate object-fit: contain.
 * Takes an object's dimension (innerWidth/innerHeight)
 * and a container's dimensions (outerWidth/outerHeight)
 * and returns a new width/height where the inner object
 * will use as much of the container's space as possible
 * while respecting the inner object's aspect ratio
 */

function contain(
  innerWidth,
  innerHeight,
  outerWidth,
  outerHeight
) {
  const innerRatio = innerWidth / innerHeight
  const outerRatio = outerWidth / outerHeight

  if (innerRatio > outerRatio) {
    let scale = outerWidth / innerWidth
    return [
      outerWidth,
      innerHeight * scale
    ]
  } else {
    let scale = outerHeight / innerHeight
    return [
      innerWidth * scale,
      outerHeight
    ]
  }
}