Based on earlier iterations in https://gist.github.com/slightlyoff/4755944

connectionchange.js

class ConnectionInfo {
  constructor(media="unknown",
              className="unknown",
              classId=0) {
    this.media = media;
    this.className = className;
    this.classId = classId;
  }
}

var typeGenerator = function(type) {
  var id = 0;
  return function(className) {
    return new ConnectionInfo(type, className, ++id);
  };
};
var e = typeGenerator("ethernet");
var w = typeGenerator("wifi");
var c = typeGenerator("cellular");

var connectionClasses = {
  ethernet: [
    e("10"),
    e("100"),
    e("1000"),
    e("10000"),
    e("100000")
  ],
  wifi: [
    w("a"),
    w("b"),
    w("g"),
    w("n"),
    w("ac"), // draft
    w("ad")  // future
  ],
  cellular: [
    c("2"),
    c("2.5"),  // GPRS
    c("2.75"), // EDGE
    c("3"),    // UMTS, CDMA2000 1X
    c("3.5"),  // HSPA, EV-DO
    c("3.75"), // HSPA+, EV-DO Advanced
    c("3.9"),  // LTE
    c("4")    // LTE Advanced
  ],
  bluetooth: [1, 1.1, 1.2, 2.0, 2.1, 3.0]
};

// Example usage:

// check current connection type, adjust logic
if(navigator.connectionInfo.media == "wifi") { ... }

// subscribe to connection type updates
window.addEvenListener("connectionchange", function(e) {
  var ci = navigator.connectionInfo;

  // Adapt based on current connection type ...
  // E.g. http://developer.android.com/training/efficient-downloads/connectivity_patterns.html
  switch (ci.media) {
    case "wifi":
      runBackgroundUpdate();

    case "cellular":
      if (ci.classId > 5) { // worse than 4G
        lowerPrefetchSize();
        break;
      }
  
    default:
      // ...
      break;
  }  
});