The code is functional and modular, and it reuses code between both functions by making common functions.
```js
function convertTemperature(temperature, from, to) {
  // Check if the temperature is a valid number.
  if (!Number.isFinite(temperature)) {
    throw new Error("Invalid temperature");
  }

  // Calculate the converted temperature.
  const convertedTemperature = (temperature - from) * (to - from) / from;

  // Return the converted temperature.
  return convertedTemperature;
}

function fahrenheitToCelsius(temperature) {
  return convertTemperature(temperature, 212, 0);
}

function celsiusToFahrenheit(temperature) {
  return convertTemperature(temperature, 0, 212);
}

// Test the functions.
console.log(fahrenheitToCelsius(212)); // 100
console.log(celsiusToFahrenheit(100)); // 212
```
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