Bangle.js Development

You may also be interested in the Bangle.js

Don't have a watch?

You can work along using the Bangle.js online Emulator however you won't get access to the sensors, Bluetooth, speaker, or vibration motor!

If your watch isn't connectable by Bluetooth:

You should hopefully be connected now! If not, take a look at

If you just want to get started and have an Android phone, you can use that too! If you don't have an external keyboard then you might want to install the free 'Hackers Keyboard' app to get access to arrow keys via touchscreen.

Note: If you've used Espruino before, make sure the Upload button (middle of the screen) shows RAM underneath it. Otherwise by writing to Flash you can remove the watch's built-in menu (you can just re-add it using

On Firmwares shipped on KickStarter Bangle.js please either leave Debug Info set to Hide in Bangle.js's Settings, or update to the latest Bootloader version using

Once connected

The Web IDE is made up out of two parts - there's the black REPL on the left, and a white editor on the right.

The REPL on the left is a direct connection to the watch and executes anything you type immediately - but be careful, it uses bracket counting to detect when to execute, so:

if (true) {;

will make your watch vibrate, but:

if (true)

With Enter rather than Alt-Enter will run three commands, the last two of which will cause an error. If you write your code in a K&R style then you're unlikely to hit issues - and if you write your code on the right hand side of the IDE, the line endings are handled automatically.

When writing on the right, you can click the button in the middle of the IDE to reset Bangle.js (temporarily!) and upload the code on the right hand side. However the initial code (that works on most Espruino devices!) will fail as there's no LED on Bangle.js.

You can now try some commands:

First, let's (temporarily) reset the watch and get rid of the connectable window:



on the left-hand side of the IDE. The Bangle.js logo will be displayed.;

Will make Bangle.js vibrate, and:


Will make it beep.

Note: The majority of Bangle.js devices do not contain a piezo speaker, but instead use the vibration motor for sound. If you received your device and it doesn't make a noise when using Bangle.beep();, please update the Bootloader and Settings app via the App Loader. Afterwards you can change the Beep mode in the Settings to Vibrate.

You'll notice they return promises, so you can chain them: => {

You might want to clear the screen, in which case you can use g.clear(). A full list of Graphics commands is at:

If you want to write a message on the screen, you can use E.showMessage:

E.showMessage("Hello","A Title")

In general, Bangle.js specific functions/events are in the Bangle object, and Espruino-specific functions are in the E object

But the screen itself contains a VT100 terminal, so you can use Terminal.print and Terminal.println if you just want to log data:

Terminal.println("Hello World")

But these will scroll the screen up, which may make currently running apps look a bit strange!

It's worth noting that when you're disconnected from Bluetooth, Bangle.js will write any messages from console.log (as well as any Exceptions) to the display if Debug info is set to Show in Settings. This can be a great way of seeing if your app is failing in unexpected ways when in every day use.

Want to react to user input on the buttons? You can query the button state. BTN1 is the top button, BTN2 is the middle, BTN3 is the bottom:;

Will output true or false depending on the state of the middle button, but often we don't want to poll because running code all the time would use battery.

Instead, you can use setWatch. This sets up the hardware to watch for a button press:

setWatch(() => {
  E.showMessage("You\npressed\nthe middle\nbutton!");
  setTimeout(()=>g.clear(), 1000);
}, BTN2);

At this point the code's getting longer, so you might want to copy this to the right-hand side of the IDE and then use the Upload button

By default, this will only happpen once, but you can add {repeat:true} to the end:

setWatch(() => {;
  E.showMessage("You\npressed\nthe middle\nbutton!");
  setTimeout(()=>g.clear(), 1000);
}, BTN2, {repeat:true});

The default is to detect button press, but you can also add edge:"falling" or edge:"both" if you want to detect when the button is released.

Using the Right-hand side of the IDE

The Right-hand side of the IDE is just a syntax-highlighted editor. To send the code you have written to Bangle.js you must click the upload button:

This preprocesses the code you wrote, loading and files you required, and the uploads it to Bangle.js. The destination is written underneath, and can be changed either by pressing the down arrow, or by going to Settings and Communications:

Next Steps

You might want to check out:


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