Web Bluetooth / Link Loss (Async Await) Sample

Available in Chrome 55+ | View on GitHub | Browse Samples

The Web Bluetooth API lets websites discover and communicate with devices over the Bluetooth 4 wireless standard using the Generic Attribute Profile (GATT). It is currently partially implemented in Android M, Chrome OS, Mac, and Windows 10.

This sample illustrates the use of the Web Bluetooth API to read and write the standardized Alert Level characteristic from a nearby Bluetooth Device used to expose the current link loss alert level that determines how the device alerts when the link is lost. You may want to try this demo with the nRF Connect for Mobile App from the Google Play Store and its pre-configured Link Loss service, and check out the Link Loss (Promises) sample.

Live Output

JavaScript Snippet

var alertLevelCharacteristic;

async function onReadButtonClick() {
  try {
    log('Requesting Bluetooth Device...');
    const device = await navigator.bluetooth.requestDevice({
     // filters: [...] <- Prefer filters to save energy & show relevant devices.
        acceptAllDevices: true,
        optionalServices: ['link_loss']});

    log('Connecting to GATT Server...');
    const server = await device.gatt.connect();

    log('Getting Link Loss Service...');
    const service = await server.getPrimaryService('link_loss');

    log('Getting Alert Level Characteristic...');
    alertLevelCharacteristic = await service.getCharacteristic('alert_level');

    document.querySelector('#writeButton').disabled = false;
    log('Reading Alert Level...');
    const value = await alertLevelCharacteristic.readValue();

    log('> Alert Level: ' + getAlertLevel(value));
  } catch(error) {
    document.querySelector('#writeButton').disabled = true;
    log('Argh! ' + error);

async function onWriteButtonClick() {
  if (!alertLevelCharacteristic) {
  let value = Uint8Array.of(document.querySelector('#alertLevelValue').value);
  try {
    log('Setting Alert Level...');
    await alertLevelCharacteristic.writeValue(value);

    log('> Alert Level changed to: ' + getAlertLevel(new DataView(value.buffer)));
  } catch(error) {
    log('Argh! ' + error);

/* Utils */

const valueToAlertLevel = {
  0x00: 'No Alert',
  0x01: 'Mild Alert',
  0x02: 'High Alert'

function getAlertLevel(value) {
  let v = value.getUint8(0);
  return v + (v in valueToAlertLevel ?
      ' (' + valueToAlertLevel[v] + ')' : 'Unknown');