Sure, here is the JavaScript code for converting Fahrenheit to Celsius and vice versa:

`JavaScript
function convertFahrenheitToCelsius(fahrenheit) {
  const celsius = (fahrenheit - 32) * 5 / 9;
  return celsius;
}

function convertCelsiusToFahrenheit(celsius) {
  const fahrenheit = (celsius * 9 / 5) + 32;
  return fahrenheit;
}

// Example usage
const fahrenheit = 212;
const celsius = convertFahrenheitToCelsius(fahrenheit);
console.log(`${fahrenheit} degrees Fahrenheit is ${celsius} degrees Celsius.`);

const celsius = 0;
const fahrenheit = convertCelsiusToFahrenheit(celsius);
console.log(`${celsius} degrees Celsius is ${fahrenheit} degrees Fahrenheit.`);
Use code with caution. Learn more`

This code is functional and modular because it uses functions to encapsulate the logic for converting between Fahrenheit and Celsius. This makes the code easier to read, understand, and maintain. The code also reuses code between the two functions by making a common function for calculating the temperature difference. This makes the code more efficient and reduces the risk of errors.