Show a message to people in IE8 or lower asking them to upgrade their browser. Depends on jQuery.

badbrowser.js

(function($) {

  var BadBrowser = {

    show: function() {
      var _this = this;
      if (_this.isIE8orLower()) {
        _this.showMessage();
      }
    },

    isIE8orLower: function() {
      var version = this.getInternetExplorerVersion();
      if (version !== -1) {
        return (version < 9);
      } else {
        return false;
      }
    },

    // Returns the version of Internet Explorer or a -1
    // (indicating the use of another browser).
    getInternetExplorerVersion: function() {
      var rv = -1; // Return value assumes failure.
      if (navigator.appName == 'Microsoft Internet Explorer') {
        var ua = navigator.userAgent;
        var re = new RegExp("MSIE ([0-9]{1,}[\\.0-9]{0,})");
        if (re.exec(ua) !== null) rv = parseFloat(RegExp.$1);
      }
      return rv;
    },

    showMessage: function() {
      var html = $('<div/>', {
        id: 'unsupported',
        html: 'Your browser is unsupported on this site. Please <a style="text-decoration: underline" href="http://browsehappy.com/" target="_blank">upgrade</a> for the best experience.',
        css: {
          "z-index": "1001",
          "border-bottom": "#d4c790 2px solid",
          "position": "fixed",
          "text-align": "center",
          "line-height": "40px",
          "background-color": "#faf4af",
          "width": "100%",
          "font-family": "sans-serif",
          "height": "40px",
          "font-size": "12px",
          "top": "0px",
          "left": "0px",
          "color": "#333"
        }
      });
      console.log('showing');
      $('body').prepend(html);
      $('#unsupported').show();
    }

  };

  // Define as an AMD module for loaders such as RequireJS. 
  // If define doesn't exist, then just wait for document ready and show it.
  if (typeof define === 'function' && define.amd) {
    define('badbrowser', function() {
      return BadBrowser;
    });
  } else {
    $(document).ready(function() {
      BadBrowser.show();
    });
  }

}(window.jQuery));