ESP8266 NodeMCU is the smallest & cheapest IoT Development board. Price is only starting from $2. There
When we write code in Arduino IDE we have to give pin name to set or low voltage operation.
For example, if we want to on or off relay using D0 pin, then we have to set digital write high for D0. This type of naming like D0, D1 is not supported for the ESP8266 library. NodeMCU supports the constant integer value of the pin numbers.
In the table below I have shared all the GPIO pin name and its corresponding integer value. When you will these pins, just initialize that pin name with its integer value at the top of the code.
ESP8266 Pins Constants GPIO Pins