TypeScript `instanceof` and `typeof`

retool-typescript-control-flow-analysis-3.ts

import { format as fmtDate, parseISO } from 'date-fns'


function formatDate(date: Date | string | number, format = 'dd-mm-YYYY') {
  let d = null
  if (typeof date === 'string') {
    date
    // `date` is of type `string`
    
    d = parseISO(date)
  } else if (typeof date === 'number') {
    // `date` is of type `number`
    
    d = new Date(date)
  } else if (date instanceof Date) {
    // `date` is of type `Date`

    d = date
  } else {
    return null
  }
  if (d.toString() === 'Invalid Date') {
    return null
  } else {
    return fmtDate(d, format)
  }
}