keep element's ratio

js

// http://codepen.io/lzl124631x/pen/xOmJGE
$.fn.keepRatio = function(ratio) {
  var self = this,
    $p = this.parent(),
    keep = function() {
      var w = $p.width(),
        h = $p.height();
      if (w / h > ratio) {
        self.height(h);
        self.width(h * ratio);
      } else {
        self.width(w);
        self.height(w / ratio);
      }
    };
  $p.on('resize', keep);
  keep();
};