// Utility function to create custom errors
const createError = name => class extends Error {
  constructor(message) {
    super(message);
    this.name = name;
  }
};

// Custom error types
const UnknownOrUnexpectedOptionError = createError('UnknownOrUnexpectedOptionError');
const MissingOptionArgumentError = createError('MissingOptionArgumentError');
// …

// And thrown this way
throw new UnknownOrUnexpectedOptionError(`Unknown or unexpected option: ${originalArgName}`);

const arg = require('arg');
const { UnknownOrUnexpectedOptionError } = arg;

try {
  const args = arg({ /* … */ });
} catch (err) {
  if (err instanceof UnknownOrUnexpectedOptionError) {
    // handle the error gracefully
  }
}