Domain Hierarchies

Domains can nest. A child domain inherits its place in the tree, so a handler can match a broad parent without enumerating every child’s ids.

Creating child domains

const err_app = defineNiceError({ domain: "err_app", schema: {} } as const);

const err_auth = err_app.createChildDomain({
  domain: "err_auth",
  schema: { /* ... */ },
});

const err_auth_registration = err_auth.createChildDomain({
  domain: "err_auth_registration",
  schema: { /* ... */ },
});

Ancestry checks

err_app.isParentOf(err_auth_registration);   // true
err_auth.isParentOf(err_auth_registration);  // true
err_auth_registration.isParentOf(err_auth);  // false

Type guards

// Exact domain match only
err_auth.isExact(someError);        // true only for the err_auth domain

// This domain OR any descendant
err_auth.isThisOrChild(someError);  // true for err_auth and all its children

Use isExact when you want to handle exactly one domain’s errors; use isThisOrChild to catch a whole subtree — e.g. “any auth-related error” regardless of which specific child domain produced it.

When to nest

• Group errors by bounded context: err_app → err_auth → err_auth_registration.
• Let middleware match broadly (err_auth.isThisOrChild) while leaf code matches precisely (err_auth_registration.isExact).
• Keep domain strings stable — they’re part of the serialized identity.

Next: Serialization & transport →