code:

interface Banana {
   kind: "banana",
   color: string,
   peeled: boolean
};
interface Apple {
   kind: "apple",
   color: string,
   leaves: number
};
function examineFruit(fruit: Apple | Banana) {
   console.log(fruit.kind);
   console.log(fruit.color); // valid
   // console.log(fruit.peeled); // invalid; not all members of `Apple | Banana` have a `peeled` member
   if (fruit.kind == "banana") {
      console.log(fruit.peeled); // valid; fruit is now inferred to be `Banana`
   }
}