memory effect on TypeScript's type safety

memeff.ts

interface Foo {
  bar: undefined | Bar
}

interface Bar {
  baz: string
}

export function getbaz(foo: Foo) {
  if (foo.bar) {
    somewrite(foo)
    return foo.bar.baz // XXX will result in error
  }
  return undefined
}

function somewrite(foo: Foo) {
  if (Math.random() < 0.5) {
    foo.bar = undefined
  }
}

const foo = {bar: {baz: "hi"}}
console.log(getbaz(foo))