TypeScript in操作符缩小

语法：

"valuse" in x

实例：

type Fish = { swim: () => void }
type Bird = { fly: () => void }
type Human = { swim?: () => void; fly?: () => void }

type Animal = Fish | Bird | Human

function move(animal: Animal) {
    if ("swim" in animal) {
        // animal: Fish | Human
        return (animal as Fish).swim();
    }
    return (animal as Bird).fly();
}