Communicating with Bluetooth devices over JavaScript

What if I told you websites could communicate with nearby Bluetooth devices
in a secure and privacy-preserving way? This way, heart rate monitors, singing
lightbulbs, and even turtles could interact directly with a website.
Until now, the ability to in…


This content originally appeared on web.dev and was authored by François Beaufort

What if I told you websites could communicate with nearby Bluetooth devices in a secure and privacy-preserving way? This way, heart rate monitors, singing lightbulbs, and even turtles could interact directly with a website.

Until now, the ability to interact with Bluetooth devices has been possible only for platform-specific apps. The Web Bluetooth API aims to change this and brings it to web browsers as well.

Before we start

This article assumes you have some basic knowledge of how Bluetooth Low Energy (BLE) and the Generic Attribute Profile (GATT) work.

Even though the Web Bluetooth API specification is not finalized yet, the spec authors are actively looking for enthusiastic developers to try out this API and give feedback on the spec and feedback on the implementation.

A subset of the Web Bluetooth API is available in Chrome OS, Chrome for Android 6.0, Mac (Chrome 56) and Windows 10 (Chrome 70). This means you should be able to request and connect to nearby Bluetooth Low Energy devices, read/write Bluetooth characteristics, receive GATT Notifications, know when a Bluetooth device gets disconnected, and even read and write to Bluetooth descriptors. See MDN's Browser compatibility table for more information.

For Linux and earlier versions of Windows, enable the #experimental-web-platform-features flag in chrome://flags.

Available for origin trials

In order to get as much feedback as possible from developers using the Web Bluetooth API in the field, Chrome has previously added this feature in Chrome 53 as an origin trial for Chrome OS, Android, and Mac.

The trial has successfully ended in January 2017.

Security requirements

To understand the security tradeoffs, I recommend the Web Bluetooth Security Model post from Jeffrey Yasskin, a software engineer on the Chrome team, working on the Web Bluetooth API specification.

HTTPS only

Because this experimental API is a powerful new feature added to the web, it is made available only to secure contexts. This means you'll need to build with TLS in mind.

User gesture required

As a security feature, discovering Bluetooth devices with navigator.bluetooth.requestDevice must be triggered by a user gesture such as a touch or a mouse click. We're talking about listening to pointerup, click, and touchend events.

button.addEventListener('pointerup', function(event) {
  // Call navigator.bluetooth.requestDevice
});

Get into the code

The Web Bluetooth API relies heavily on JavaScript Promises. If you're not familiar with them, check out this great Promises tutorial. One more thing, () => {} are simply ECMAScript 2015 Arrow functions.

Request Bluetooth devices

This version of the Web Bluetooth API specification allows websites, running in the Central role, to connect to remote GATT Servers over a BLE connection. It supports communication among devices that implement Bluetooth 4.0 or later.

When a website requests access to nearby devices using navigator.bluetooth.requestDevice, the browser prompts user with a device chooser where they can pick one device or simply cancel the request.

Bluetooth device user prompt.

The navigator.bluetooth.requestDevice function takes a mandatory object that defines filters. These filters are used to return only devices that match some advertised Bluetooth GATT services and/or the device name.

For instance, to request Bluetooth devices advertising the Bluetooth GATT Battery Service:

navigator.bluetooth.requestDevice({ filters: [{ services: ['battery_service'] }] })
.then(device => { /* … */ })
.catch(error => { console.error(error); });

If your Bluetooth GATT Service is not on the list of the standardized Bluetooth GATT services though, you may provide either the full Bluetooth UUID or a short 16- or 32-bit form.

navigator.bluetooth.requestDevice({
  filters: [{
    services: [0x1234, 0x12345678, '99999999-0000-1000-8000-00805f9b34fb']
  }]
})
.then(device => { /* … */ })
.catch(error => { console.error(error); });

You can also request Bluetooth devices based on the device name being advertised with the name filters key, or even a prefix of this name with the namePrefix filters key. Note that in this case, you will also need to define the optionalServices key to be able to access some services. If you don't, you'll get an error later when trying to access them.

navigator.bluetooth.requestDevice({
  filters: [{
    name: 'Francois robot'
  }],
  optionalServices: ['battery_service']
})
.then(device => { /* … */ })
.catch(error => { console.error(error); });

Finally, instead of filters you can use the acceptAllDevices key to show all nearby Bluetooth devices. You will also need to define the optionalServices key to be able to access some services. If you don't, you'll get an error later when trying to access them.

navigator.bluetooth.requestDevice({
  acceptAllDevices: true,
  optionalServices: ['battery_service']
})
.then(device => { /* … */ })
.catch(error => { console.error(error); });

Caution: This may result in a bunch of unrelated devices being shown in the chooser and energy being wasted as there are no filters. Use it with caution.

Connect to a Bluetooth device

So what do you do now that you have a BluetoothDevice? Let's connect to the Bluetooth remote GATT Server which holds the service and characteristic definitions.

navigator.bluetooth.requestDevice({ filters: [{ services: ['battery_service'] }] })
.then(device => {
  // Human-readable name of the device.
  console.log(device.name);

  // Attempts to connect to remote GATT Server.
  return device.gatt.connect();
})
.then(server => { /* … */ })
.catch(error => { console.error(error); });

Read a Bluetooth Characteristic

Here we are connected to the GATT Server of the remote Bluetooth device. Now we want to get a Primary GATT Service and read a characteristic that belongs to this service. Let's try, for instance, to read the current charge level of the device's battery.

In the example below, battery_level is the standardized Battery Level Characteristic.

navigator.bluetooth.requestDevice({ filters: [{ services: ['battery_service'] }] })
.then(device => device.gatt.connect())
.then(server => {
  // Getting Battery Service…
  return server.getPrimaryService('battery_service');
})
.then(service => {
  // Getting Battery Level Characteristic…
  return service.getCharacteristic('battery_level');
})
.then(characteristic => {
  // Reading Battery Level…
  return characteristic.readValue();
})
.then(value => {
  console.log(`Battery percentage is ${value.getUint8(0)}`);
})
.catch(error => { console.error(error); });

If you use a custom Bluetooth GATT characteristic, you may provide either the full Bluetooth UUID or a short 16- or 32-bit form to service.getCharacteristic.

Note that you can also add a characteristicvaluechanged event listener on a characteristic to handle reading its value. Check out the Read Characteristic Value Changed Sample to see how to optionally handle upcoming GATT notifications as well.

…
.then(characteristic => {
  // Set up event listener for when characteristic value changes.
  characteristic.addEventListener('characteristicvaluechanged',
                                  handleBatteryLevelChanged);
  // Reading Battery Level…
  return characteristic.readValue();
})
.catch(error => { console.error(error); });

function handleBatteryLevelChanged(event) {
  const batteryLevel = event.target.value.getUint8(0);
  console.log('Battery percentage is ' + batteryLevel);
}

Write to a Bluetooth Characteristic

Writing to a Bluetooth GATT Characteristic is as easy as reading it. This time, let's use the Heart Rate Control Point to reset the value of the Energy Expended field to 0 on a heart rate monitor device.

I promise there is no magic here. It's all explained in the Heart Rate Control Point Characteristic page.

navigator.bluetooth.requestDevice({ filters: [{ services: ['heart_rate'] }] })
.then(device => device.gatt.connect())
.then(server => server.getPrimaryService('heart_rate'))
.then(service => service.getCharacteristic('heart_rate_control_point'))
.then(characteristic => {
  // Writing 1 is the signal to reset energy expended.
  const resetEnergyExpended = Uint8Array.of(1);
  return characteristic.writeValue(resetEnergyExpended);
})
.then(_ => {
  console.log('Energy expended has been reset.');
})
.catch(error => { console.error(error); });

Receive GATT notifications

Now, let's see how to be notified when the Heart Rate Measurement characteristic changes on the device:

navigator.bluetooth.requestDevice({ filters: [{ services: ['heart_rate'] }] })
.then(device => device.gatt.connect())
.then(server => server.getPrimaryService('heart_rate'))
.then(service => service.getCharacteristic('heart_rate_measurement'))
.then(characteristic => characteristic.startNotifications())
.then(characteristic => {
  characteristic.addEventListener('characteristicvaluechanged',
                                  handleCharacteristicValueChanged);
  console.log('Notifications have been started.');
})
.catch(error => { console.error(error); });

function handleCharacteristicValueChanged(event) {
  const value = event.target.value;
  console.log('Received ' + value);
  // TODO: Parse Heart Rate Measurement value.
  // See https://github.com/WebBluetoothCG/demos/blob/gh-pages/heart-rate-sensor/heartRateSensor.js
}

The Notifications Sample shows you to how to stop notifications with stopNotifications() and properly remove the added characteristicvaluechanged event listener.

Disconnect from a Bluetooth Device

To provide a better user experience, you may want to listen for disconnection events and invite the user to reconnect:

navigator.bluetooth.requestDevice({ filters: [{ name: 'Francois robot' }] })
.then(device => {
  // Set up event listener for when device gets disconnected.
  device.addEventListener('gattserverdisconnected', onDisconnected);

  // Attempts to connect to remote GATT Server.
  return device.gatt.connect();
})
.then(server => { /* … */ })
.catch(error => { console.error(error); });

function onDisconnected(event) {
  const device = event.target;
  console.log(`Device ${device.name} is disconnected.`);
}

You can also call device.gatt.disconnect() to disconnect your web app from the Bluetooth device. This will trigger existing gattserverdisconnected event listeners. Note that it will NOT stop bluetooth device communication if another app is already communicating with the Bluetooth device. Check out the Device Disconnect Sample and the Automatic Reconnect Sample to dive deeper.

Caution: Bluetooth GATT attributes, services, characteristics, etc. are invalidated when a device disconnects. This means your code should always retrieve (through getPrimaryService(s), getCharacteristic(s), etc.) these attributes after reconnecting.

Read and write to Bluetooth descriptors

Bluetooth GATT descriptors are attributes that describe a characteristic value. You can read and write them to in a similar way to Bluetooth GATT characteristics.

Let's see for instance how to read the user description of the measurement interval of the device's health thermometer.

In the example below, health_thermometer is the Health Thermometer service, measurement_interval the Measurement Interval characteristic, and gatt.characteristic_user_description the Characteristic User Description descriptor.

navigator.bluetooth.requestDevice({ filters: [{ services: ['health_thermometer'] }] })
.then(device => device.gatt.connect())
.then(server => server.getPrimaryService('health_thermometer'))
.then(service => service.getCharacteristic('measurement_interval'))
.then(characteristic => characteristic.getDescriptor('gatt.characteristic_user_description'))
.then(descriptor => descriptor.readValue())
.then(value => {
  const decoder = new TextDecoder('utf-8');
  console.log(`User Description: ${decoder.decode(value)}`);
})
.catch(error => { console.error(error); });

Now that we've read the user description of the measurement interval of the device's health thermometer, let's see how to update it and write a custom value.

navigator.bluetooth.requestDevice({ filters: [{ services: ['health_thermometer'] }] })
.then(device => device.gatt.connect())
.then(server => server.getPrimaryService('health_thermometer'))
.then(service => service.getCharacteristic('measurement_interval'))
.then(characteristic => characteristic.getDescriptor('gatt.characteristic_user_description'))
.then(descriptor => {
  const encoder = new TextEncoder('utf-8');
  const userDescription = encoder.encode('Defines the time between measurements.');
  return descriptor.writeValue(userDescription);
})
.catch(error => { console.error(error); });

Samples, demos and codelabs

All Web Bluetooth samples below have been successfully tested. To enjoy these samples to their fullest, I recommend you install the BLE Peripheral Simulator Android App which simulates a BLE peripheral with a Battery Service, a Heart Rate Service, or a Health Thermometer Service.

Beginner

  • Device Info - retrieve basic device information from a BLE Device.
  • Battery Level - retrieve battery information from a BLE Device advertising Battery information.
  • Reset Energy - reset energy expended from a BLE Device advertising Heart Rate.
  • Characteristic Properties - display all properties of a specific characteristic from a BLE Device.
  • Notifications - start and stop characteristic notifications from a BLE Device.
  • Device Disconnect - disconnect and get notified from a disconnection of a BLE Device after connecting to it.
  • Get Characteristics - get all characteristics of an advertised service from a BLE Device.
  • Get Descriptors - get all characteristics' descriptors of an advertised service from a BLE Device.

Combining multiple operations

Check out our curated Web Bluetooth Demos and official Web Bluetooth Codelabs as well.

Libraries

  • web-bluetooth-utils is a npm module that adds some convenience functions to the API.
  • A Web Bluetooth API shim is available in noble, the most popular Node.js BLE central module. This allows you to webpack/browserify noble without the need for a WebSocket server or other plugins.
  • angular-web-bluetooth is a module for Angular that abstracts away all the boilerplate needed to configure the Web Bluetooth API.

Tools

  • Get Started with Web Bluetooth is a simple Web App that will generate all the JavaScript boilerplate code to start interacting with a Bluetooth device. Enter a device name, a service, a characteristic, define its properties and you're good to go.
  • If you're already a Bluetooth developer, the Web Bluetooth Developer Studio Plugin will also generate the Web Bluetooth JavaScript code for your Bluetooth device.

Tips

A Bluetooth Internals page is available in Chrome at chrome://bluetooth-internals so that you can inspect everything about nearby Bluetooth devices: status, services, characteristics, and descriptors.

Screenshot of the internal page to debug Bluetooth in Chrome
Internal page in Chrome for debugging Bluetooth devices.

I also recommend checking out the official How to file Web Bluetooth bugs page as debugging Bluetooth can be hard sometimes.

What's next

Check the browser and platform implementation status first to know which parts of the Web Bluetooth API are currently being implemented.

Though it's still incomplete, here's a sneak peek of what to expect in the near future:

  • Scanning for nearby BLE advertisements will happen with navigator.bluetooth.requestLEScan().
  • A new serviceadded event will track newly discovered Bluetooth GATT Services while serviceremoved event will track removed ones. A new servicechanged event will fire when any characteristic and/or descriptor gets added or removed from a Bluetooth GATT Service.

Resources

Acknowledgements

Thanks to Kayce Basques for reviewing this article. Hero image by SparkFun Electronics from Boulder, USA.


This content originally appeared on web.dev and was authored by François Beaufort


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François Beaufort | Sciencx (2015-07-21T00:00:00+00:00) Communicating with Bluetooth devices over JavaScript. Retrieved from https://www.scien.cx/2015/07/21/communicating-with-bluetooth-devices-over-javascript/

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