Not sure if this is the best use case for an IIFE, but it made it simpler than writing a lot of flow-checks in line:

if (
    route.name === MessageListRouteName ||
    (typeof route.params?.presentedFrom === 'string' &&
      route.params.presentedFrom.startsWith(MessageListRouteName))
)

Separately, I check presentedFrom.startsWith instead of presentedFrom === since the presentedFrom value is the navigation key, not just the route name. So it'll be something like MessageList83918

What's the downside of using the IIFE here though?

The IIFE is also going to get created/called on every re-render. I feel like it would be simpler/easier to understand to instead break this down into two separate expressions, maybe something like:

const validType = typeof route.params?.presentedFrom === 'string'; // not the best variable name here
const presentedFromMessageList = validType && route.params.presentedFrom.startsWith(MessageListRouteName);

Ok that's fair, I only went down this route since I thought it was slightly cleaner. I'll update this