https://stackoverflow.com/questions/3971841/how-to-resize-images-proportionally-keeping-the-aspect-ratio

resize.js

 /**
  * Conserve aspect ratio of the original region. Useful when shrinking/enlarging
  * images to fit into a certain area.
  *
  * @param {Number} srcWidth width of source image
  * @param {Number} srcHeight height of source image
  * @param {Number} maxWidth maximum available width
  * @param {Number} maxHeight maximum available height
  * @return {Object} { width, height }
  */
function calculateAspectRatioFit(srcWidth, srcHeight, maxWidth, maxHeight) {
    var ratio = Math.min(maxWidth / srcWidth, maxHeight / srcHeight);
    return { width: srcWidth*ratio, height: srcHeight*ratio };
 }