Proj4js classic demo based on OpenLayers 2.x and Proj4js 1.1. Open the brower console to see the result by ThomasG77

index.html

<!doctype html>
<html lang="en">
  <head>
    <script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/proj4js/1.1.0/proj4js-compressed.min.js"></script>
    <script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/openlayers/2.13.1/OpenLayers.js"></script>
    <script type="text/javascript">
      //Correct datum for potsdam (altough already corrected with changes in towgs84 in the projection declaration)
      // Only useful if you use the outdated spatialreference.org proj4js definition whereas epsg.io is right...
      Proj4js.Datum["potsdam"] = {towgs84: "598.1,73.7,418.2,0.202,0.045,-2.455,6.7", ellipse: "bessel", datumName: "Potsdam Rauenberg 1950 DHDN"};
      // Declare projection
      Proj4js.defs["EPSG:31466"] = "+proj=tmerc +lat_0=0 +lon_0=6 +k=1 +x_0=2500000 +y_0=0 +ellps=bessel +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7 +units=m +no_defs";
      console.log(Proj4js.defs["EPSG:31466"]);
      var proj_wgs84 = new OpenLayers.Projection("EPSG:4326");
	  var proj_31466 = new OpenLayers.Projection("EPSG:31466");
	  // Center for projection EPSG:31466
	  var point = new OpenLayers.LonLat(2769212.70, 5678724.61);
	  var new_point= point.clone().transform(proj_31466, proj_wgs84);
      console.log(new_point.lat, new_point.lon);
    </script>
  </head>
  <body>
  </body>
</html>