NodeMCU esp8266 VCC measurements w/ correction for on-board voltage divider
ESP8266 has ADC which can measure internal voltage reference (TODO: Find specs for this and schematic).
ADC is also attached to A0 pin, and this in turn is attached to a voltage dividor circuit on the board which interferes when using ESP8266 in ADC_VCC mode, so we must compensate with magic numbers based on resistor network that is in place.
Your numbers may vary, calibrate known regulated voltage to match by changing magic numbers.
Value of 898.00f gives 3.30v reading on my board. YMMV