How can you return a string if isCustomDate only accepts a Boolean

[FractalMind]

isCustomDate only accepts a Boolean

ngx-daterangepicker-material/src/daterangepicker/daterangepicker.component.ts

Line 368 in e874ad4

isCustomDate(date: Dayjs): boolean {

In the doc it says:

isCustomDate
(function) A function that is passed each date in the calendars before they are displayed, and may return a string or array of CSS class names to apply to that date's calendar cell

How can you return a string or an array of string?