NodeMCU is an open source Lua based firmware for the ESP32 and ESP8266 WiFi SOCs from Espressif. It uses an on-module flash-based SPIFFS file system. NodeMCU is implemented in C and the ESP32 version 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.
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The NodeMCU [`release`](https://github.com/nodemcu/nodemcu-firmware/tree/release) and [`dev`](https://github.com/nodemcu/nodemcu-firmware/tree/dev) branches target the ESP8266. The [`dev-esp32`](https://github.com/nodemcu/nodemcu-firmware/tree/dev-esp32) branch targets the ESP32.
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.