code: function isValidID(id) {
    return id === '' || (
      id.charAt(0) === SEPARATOR && id.charAt(id.length - 1) !== SEPARATOR
    );
  }
docs: Checks if the supplied string is a valid React DOM ID.
  
  @param {string} id A React DOM ID, maybe.
  @return {boolean} True if the string is a valid React DOM ID.
  @private