NodeMCU is an open source Lua based firmware for the ESP32 and ESP8266 WiFi SOC from Espressif and uses an on-module flash-based SPIFFS file system. NodeMCU is implemented in C and is layered on the Espressif ESP-IDF.
The firmware was initially developed as is a companion project to the popular ESP8266-based NodeMCU development modules, but the project is now community-supported, and the firmware can now be run on any ESP module.
Support for the new ESP32 WiFi/BlueTooth SOC from Espressif is under way.
At the moment the only up-to-date documentation maintained by the current NodeMCU team is in English. It is part of the source code repository (/docs subfolder) and kept in sync with the code.