TypeScript type guard for ensuring / asserting specific optional property

index.ts

/** Interface with optional properties */
interface IOptionalData {
  foo?: { bar?: string };
  other?: { bar?: string};
  always: { bar?: string };
}

/** Make certain keys of a type required */
type RequiredKeys<T, K extends keyof T> = Exclude<T, K> & Required<Pick<T, K>>

/** Typeguard for property 'foo' in IOptionalData */
const ensureFooProperty = (data: IOptionalData): data is RequiredKeys<IOptionalData, 'foo'> =>
  !!data.foo && typeof data.foo.bar === 'string'

const accessData = (data: IOptionalData) => {
  if (ensureFooProperty(data)) {
    console.log(data.always.bar) // always is always defined
    console.log(data.other.bar)  // COMPILER ERROR: 'other' is possibly undefined
    return data.foo.bar          // accessing optional props is allowed due to ensureToFoo
  }
  console.log(data.foo.bar)      // COMPILER ERROR: 'foo' is possibly undefined
}