This example blinks an external LED connected to the D0 pin of the NodeMCU.

void setup() {
  pinMode(D0, OUTPUT);  
}

void loop() {
  digitalWrite(D0, HIGH);   
  delay(1000);               
  digitalWrite(D0, LOW);    
  delay(1000);               
}

The LED is connected to D0 through a resistor (100–220Ω) and then to ground.