If you want to disconnect to save power, I'd recommend that you actually bring the CH_PD wire out from the ESP8266 and connect it to Espruino - you can then totally power the chip down, which should be a lot more efficient than just telling it to disconnect from WiFi.
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.
There actually isn't a disconnect function in the ESP8266_WiFi driver - there's just what is mentioned here: http://www.espruino.com/ESP8266#reference
If you want to disconnect to save power, I'd recommend that you actually bring the CH_PD wire out from the ESP8266 and connect it to Espruino - you can then totally power the chip down, which should be a lot more efficient than just telling it to disconnect from WiFi.