For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt, and this page is available as Markdown at /reference/control/isErr.md.
Install massaman

    isErr

    Type guard that narrows a Result to Err.

    if (isErr(result)) {
  // result.error is Error
}

    Usage

    isErr(result)

    The inverse of isOk. Useful when the failure path is the early-return: check isErr, bail, continue with the narrowed Ok after.

    import { attemptAsync, isErr } from 'massaman'

const user = await attemptAsync(() => db.users.findById(id))
if (isErr(user)) {
  return respond.serverError(user.error)
}
// user is Ok<User> here
return respond.ok(user.value)

    Parameters

    resultResult<T>required

    the result to test.

    Returns

    true if the result is a failure.

    result is Err

    See also