Web Bluetooth / Notifications Sample

var myCharacteristic;  function onStartButtonClick() {   let serviceUuid = document.querySelector('#service').value;   if (serviceUuid.startsWith('0x')) {     serviceUuid = parseInt(serviceUuid);   }    let characteristicUuid = document.querySelector('#characteristic').value;   if (characteristicUuid.startsWith('0x')) {     characteristicUuid = parseInt(characteristicUuid);   }    log('Requesting Bluetooth Device...');   navigator.bluetooth.requestDevice({filters: [{services: [serviceUuid]}]})   .then(device => {     log('Connecting to GATT Server...');     return device.gatt.connect();   })   .then(server => {     log('Getting Service...');     return server.getPrimaryService(serviceUuid);   })   .then(service => {     log('Getting Characteristic...');     return service.getCharacteristic(characteristicUuid);   })   .then(characteristic => {     myCharacteristic = characteristic;     return myCharacteristic.startNotifications().then(_ => {       log('> Notifications started');       myCharacteristic.addEventListener('characteristicvaluechanged',           handleNotifications);     });   })   .catch(error => {     log('Argh! ' + error);   }); }  function onStopButtonClick() {   if (myCharacteristic) {     myCharacteristic.stopNotifications()     .then(_ => {       log('> Notifications stopped');       myCharacteristic.removeEventListener('characteristicvaluechanged',           handleNotifications);     })     .catch(error => {       log('Argh! ' + error);     });   } }  function handleNotifications(event) {   let value = event.target.value;   let a = [];   // Convert raw data bytes to hex values just for the sake of showing something.   // In the "real" world, you'd use data.getUint8, data.getUint16 or even   // TextDecoder to process raw data bytes.   for (let i = 0; i < value.byteLength; i++) {     a.push('0x' + ('00' + value.getUint8(i).toString(16)).slice(-2));   }   log('> ' + a.join(' ')); }