function toENV(key) {
  let str = '';
  let previousIsUpper = false;
  for (let i = 0; i < key.length; i++) {
    const char = key[i];
    if (letters.indexOf(char) >= 0) {
      if (!previousIsUpper) {
        str += '_';
        previousIsUpper = true;
      }
    } else {
      previousIsUpper = false;
    }
    str += char;
  }
  return str.toUpperCase();
}
The prefix of environment variables for nested options.