let d;
d = 10;
d = "333";
d = true;

let s: string;
s = d; //any值可以赋给其他不同类型

let e: unknown;
// s = e; unknown不能赋值其他

if(typeof e === "string"){
    s = e;  //方法1.先判断
}
s = e as string //方法2.类型断言
s = <string>e //另外写法