Pass a defaultVersion option to app.enableVersioning(). Unversioned requests — those without a version prefix or header — are routed to the default version's handlers. This is useful during migrations when existing clients do not yet send a version identifier. VERSION_NEUTRAL routes always match regardless of defaultVersion.
defaultVersion prevents breaking existing clients during a migration to versioned APIs.
Clients that send no version header or prefix receive the default version's responses transparently.
VERSION_NEUTRAL routes bypass the defaultVersion setting — they match regardless of version context.
Set defaultVersion to the latest stable version once all clients have been migrated.
Passing an array to defaultVersion tries each in order — useful when two versions are considered current.