someComponent.jsx

const SomeComponent = ({ items = [], todaysDate, tomorrowsDate }) => {
  const [someState, setSomeState] = useState(null)
  
  return (
    <div>
      <h2>Today is {todaysDate}</h2>
      <small>And tomorrow is {tomorrowsDate}</small>
      <hr />
      {items.map((item, index) => (
        <span key={`item_${index}`}>{item.email}</span>
      ))}
    </div>
  )
}

const App = ({ dates, ...otherProps }) => {
  let items
  if (dates) {
    items = dates ? dates.map((d) => new Date(d).toLocaleDateString()) : null
  }
  
  return (
    <div>
      <SomeComponent {...otherProps} items={items} />
    </div>
  )
}

I think you made a mistake having the dates ternary inside if(dates)