Preserve aspect ratio while resizing?

When a user resizes an image, I'm trying to maintain the original aspect ratio, as there's little need to skew an image. I believe I've found the code handling this behavior. However, it's unclear to me whether I should modify the Start key or something else.

@artf - Any suggestions?

     * Start resizing
     * @param  {Event} e
     */

  }, {
    key: 'start',
    value: function start(e) {
      //Right or middel click
      if (e.button !== 0) {
        return;
      }
      e.preventDefault();
      e.stopPropagation();
      var el = this.el;
      var resizer = this;
      var config = this.opts || {};
      var attrName = 'data-' + config.prefix + 'handler';
      var rect = this.getElementPos(el);
      this.handlerAttr = e.target.getAttribute(attrName);
      this.clickedHandler = e.target;
      this.startDim = {
        t: rect.top,
        l: rect.left,
        w: rect.width,
        h: rect.height
      };
      this.rectDim = {
        t: rect.top,
        l: rect.left,
        w: rect.width,
        h: rect.height
      };
      this.startPos = {
        x: e.clientX,
        y: e.clientY
      };

      // Listen events
      var doc = this.getDocumentEl();
      (0, _mixins.on)(doc, 'mousemove', this.move);
      (0, _mixins.on)(doc, 'keydown', this.handleKeyDown);
      (0, _mixins.on)(doc, 'mouseup', this.stop);
      (0, _underscore.isFunction)(this.onStart) && this.onStart(e, { docs: doc, config: config, el: el, resizer: resizer });
      this.move(e);
    }

I'm looking to integrate this helper function:

 /**
  * Conserve aspect ratio of the orignal region. 
  *
  * @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 };
 }