TypeScript truthiness narrowing

retool-typescript-control-flow-analysis-1.ts

interface Person {
  name: string
  address?: {
    city: string
    zipcode: string
  }
}

function getCity(p: Person): string | undefined {
  if (p.address) {
    p.address 
    // `p.address` is of type `{ city: string; zipcode: string; }`
    return p.address.city
  }

  p.address 
  // `p.address` is of type `{ city: string; zipcode: string; } | undefined`

  return p.address
}