code: function zoom(ratio) {
      var hasTooltip = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : false;
  
      var _originalEvent = arguments.length > 2 && arguments[2] !== undefined ? arguments[2] : null;
  
      var imageData = this.imageData;
  
  
      ratio = Number(ratio);
  
      if (ratio < 0) {
        ratio = 1 / (1 - ratio);
      } else {
        ratio = 1 + ratio;
      }
  
      this.zoomTo(imageData.width * ratio / imageData.naturalWidth, hasTooltip, _originalEvent);
  
      return this;
    }
docs: Zoom the image with a relative ratio.
  @param {number} ratio - The target ratio.
  @param {boolean} [hasTooltip=false] - Indicates if it has a tooltip or not.
  @param {Event} [_originalEvent=null] - The original event if any.
  @returns {Viewer} this