Running more than one function in Node.js

I am having trouble understanding why my application will terminate after running only one function. I have coded the application as follows...

const readline = require('readline');

const io = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

console.log(
  'Welcome. This application will check if a given year is a leap year.'
);

checkYear();

function checkYear() {
   io.question('Check which year? ', function (year) {
      year % 100 === 0 && year % 400 !== 0
         ? false
         : year % 4 === 0
         ? console.log(year + ' is a leap year.')
         : console.log(year + ' is not a leap year.');
      });

   io.question('Check another year? y/n'), function(response) {
      if (response.toLowerCase === 'y') {
         checkYear();
      } else {
         io.close();
      }
   }
};

I have also tried breaking the second prompt into its own function and calling both checkYear and checkAgain back-to-back, but I get the same result. What do I need to do to make the io.question('Check another year') prompt run after the application responds to the first input?

I believe you are expecting another input with 2nd io.question call.
In your code, 2nd io.question will be executed immediately after the 1st io.question without waiting for user to enter anything.

So, you will need to nest the 2nd call inside the 1st call:

function checkYear() {
  io.question("Check which year? ", function (year) {
    year % 100 === 0 && year % 400 !== 0
      ? false
      : year % 4 === 0
      ? console.log(year + " is a leap year.")
      : console.log(year + " is not a leap year.");

    io.question("Check another year? y/n", function (response) {
      if (response.toLowerCase() === "y") {
        checkYear();
      } else {
        io.close();
      }
    });
  });
}

Other than that, couple of typo mistakes are:

• should be

io.question("Check another year? y/n", function (response) {
...
});

NOT

io.question('Check another year? y/n'), function(response) {
...
}

• should be
  response.toLowerCase() === "y"
  NOT
  response.toLowerCase === 'y'

You can refer to my sandbox: https://codesandbox.io/p/sandbox/friendly-lake-b15kkz
Hope this helps