If you want to have just a single app that always runs, maybe just set it up as a clock face (http://www.espruino.com/Bangle.js+Clock) and then delete the default clock and you'll have your app always running - but can still open the launcher and start whatever apps you want
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
As @Ganblejs says really. You'll want to reinstall the bootloader app to get rid of the code that's currently saved to flash, and then you can write to a Storage file instead and all the existing Bangle.js code will be there to help out - but hopefully http://www.espruino.com/Bangle.js+Development (linked above) or http://www.espruino.com/Bangle.js+First+App#making-an-app should be a good start.
If you want to have just a single app that always runs, maybe just set it up as a clock face (http://www.espruino.com/Bangle.js+Clock) and then delete the default clock and you'll have your app always running - but can still open the launcher and start whatever apps you want