This is the value that was passed to the Error() constructor in the options.cause argument.

The value can be of any type. You should not make assumptions that the error you caught has an Error as its cause, in the same way that you cannot be sure the variable bound in the catch statement is an Error either. The "Providing structured data as the error cause" example below shows a case where a non-error is deliberately provided as the cause.

It is sometimes useful to catch an error and re-throw it with a new message. In this case you should pass the original error into the constructor for the new Error, as shown.

try {
  connectToDatabase();
} catch (err) {
  throw new Error('Connecting to database failed.', { cause: err });
}

For a more detailed example see Error > Differentiate between similar errors.

Error messages written for human consumption may be inappropriate for machine parsing — since they're subject to rewording or punctuation changes that may break any existing parsing written to consume them. So when throwing an error from a function, as an alternative to a human-readable error message, you can instead provide the cause as structured data, for machine parsing.

function makeRSA(p, q) {
  if (!Number.isInteger(p) || !Number.isInteger(q)) {
    throw new Error('RSA key generation requires integer inputs.', {
      cause: { code: 'NonInteger', value: [p, q] },
    });
  }
  if (!areCoprime(p, q)) {
    throw new Error('RSA key generation requires two co-prime integers.', {
      cause: { code: 'NonCoprime', values: [p, q] },
    })
  }
  // rsa algorithm…
}

• Error.prototype.message
• Error.prototype.toString()