Add an error code to the unkown option error

[matheuss]

Before:

let args
try {
  args = arg(...)
} catch (err) {
  if (err.message.startsWith('Unknown or unexpected option:')) {
    // print a nice error
  }
}

After:

let args
try {
  args = arg(...)
} catch (err) {
  if (err.code === 'UNKNOWN_OPTION') {
    // print a nice error
  }
}

[Qix-]

I'm 👎 on this since IMO .code is reserved for errno codes.

However, including the parser state in the error message as properties wouldn't be the worst.

[bpierre]

Another approach would be to export custom error types.

In arg:

// 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}`);

It would then make it easy to know what error is being thrown:

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

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

[rauchg]

.code is not reserved. Node.js just uses it.

@matheuss can you:

• rename the code to ARG_UNKNOWN_OPTION (prefixing it with the project name seems sensible)
• add a note about this in the README

[rauchg]

#8 (comment)

Done