temperature.js

export const scales = {
  C: { label: 'Celsius (°C)', toK: value => value + 273.15, fromK: kelvin => kelvin - 273.15, symbol: '°C' },
  F: { label: 'Fahrenheit (°F)', toK: value => (value + 459.67) * 5 / 9, fromK: kelvin => kelvin * 9 / 5 - 459.67, symbol: '°F' },
  K: { label: 'Kelvin (K)', toK: value => value, fromK: kelvin => kelvin, symbol: 'K' },
  R: { label: 'Rankine (°R)', toK: value => value * 5 / 9, fromK: kelvin => kelvin * 9 / 5, symbol: '°R' }
};

export function convertTemperature(value, from, to) {
  if (!Number.isFinite(value)) throw new TypeError('value must be finite');
  if (!scales[from] || !scales[to]) throw new RangeError('unknown temperature scale');
  return scales[to].fromK(scales[from].toK(value));
}

if (typeof document !== 'undefined') {
  const value = document.querySelector('#value');
  const from = document.querySelector('#from');
  const to = document.querySelector('#to');
  const result = document.querySelector('#result');

  for (const [key, scale] of Object.entries(scales)) {
    from.add(new Option(scale.label, key));
    to.add(new Option(scale.label, key));
  }
  from.value = 'C';
  to.value = 'F';

  function render() {
    const numericValue = Number(value.value);
    if (!Number.isFinite(numericValue)) {
      result.textContent = 'Enter a numeric value';
      return;
    }
    const converted = convertTemperature(numericValue, from.value, to.value);
    result.textContent = `${numericValue} ${scales[from.value].symbol} = ${converted.toLocaleString(undefined, { maximumFractionDigits: 10 })} ${scales[to.value].symbol}`;
  }

  for (const element of [value, from, to]) element.addEventListener('input', render);
  render();
}