These docs are for an old release.
Info on upgrading to v6
weekNumberFormat
Controls the week number text.
Date Formatter, default: { week: 'narrow' }
The formatted text depends on weekText.
If set to { week: 'short' }, and weekText is set to "W", a value like "W 6" will be outputted.
If set to { week: 'narrow' }, and weekText is set to "W", a value like "W6" will be outputted.
If set to { week: 'numeric' }, a plain numeric value like "6" will be outputted.