Once you've uploaded code, you can inspect and change variables (including functions!) while your program is running. There's loads of documentation, tutorials and support for a huge range of different hardware too.
Note: Unlike Espruino Pico and the original Espruino board, Espruno WiFi doesn't contain any battery switchover circuitry. The +/+VUSB pin is connected straight to USB 5V, and shouldn't be used to power the WiFi board while Micro USB is plugged in, unless it is via a diode from 5V.