Write Characteristic to Web Bluetooth.

I think I answered this at JSOxford, but if anyone else is interested:

Web Bluetooth doesn't let you create services on the device with the Web Browser.

However to 'push' data from Espruino to a device with a Web Bluetooth browser, you just need to use notifications: http://www.espruino.com/About+Bluetooth+­LE#services-and-characteristics

So on Espruino you do:

NRF.setServices({
  0xBCDE : {
    0xABCD : {
      value : "Hello",
      maxLen : 5, 
      notify : true
}}});
function pushData(d) {
 NRF.updateServices({
  0xBCDE : {
    0xABCD : {
      value : d,
      notify : true
 }}});
}

And then on Web Bluetooth you get the characteristic as normal and then call startNotifications on it, which causes a characteristicvaluechanged event on it whenever you update the characteristic on Espruino.

I don't think there's any info on the Espruino site about that yet, but there is a workshop I did which has the relevant code: https://github.com/gfwilliams/workshop-t­hingy52/blob/master/step5.md