Simple code for demonstrating Digital Input and Digital Output in ESP8266 NodeMCU Board.

ESP8266-NodeMCU-Digital-Input-Output.ino

#define buttonPin 4 /* GPIO4 (D2) for Push Button */
#define ledPin 5 /* GPIO5 (D1) for LED */

int ledState = LOW; /* Variable to hold the current state of LED Pin. Initialized to LOW */
int buttonState = LOW; /* Variable to hold current state of Button Pin. Initialized to LOW */
int buttonValue; /* Variable to store state of the Button */
int lastButtonState = LOW; /* Variable to hold the previous state of the Button. Initialized to LOW */

long lastDebounceTime = 0; /* Variable to hold the last time the LED Pin was toggled */
long debounceDelay = 50; /* Debounce Time */

void setup()
{
  pinMode(buttonPin, INPUT); /* Initialize Button Pin as Output */
  pinMode(ledPin, OUTPUT); /* Initialize LED Pin as Input */
}

void loop()
{
  buttonValue = digitalRead(buttonPin); /* Read the state of the Button into the variable: buttonValue */

  /* Reset the debounce timer after button press */
  if(buttonValue != lastButtonState)
  {
    lastDebounceTime = millis();
  }

  /* Use the button state after waiting for debouncing */ 
  if((millis() - lastDebounceTime) > debounceDelay)
  {
    if(buttonValue != buttonState) /* Check if the button state has changed */
    {
      buttonState = buttonValue;
      if(buttonState == HIGH) /* If the button state is HIGH, toggle the LED state */
      {
        ledState = !ledState;
      }
    }
  }
  digitalWrite(ledPin, ledState); /* Set the new state of the LED */
  lastButtonState = buttonValue; /* Store the present button state for next loop */ 
}