类型别名

类型别名用来给一个类型起个新名字。举个例子：

type Name = string;
type NameResolver = () => string;
type NameOrResolver = Name | NameResolver
function getName(n: NameOrResolver) : Name {
    if (typeof n === 'string') {
        return n;
    } else {
        return n()
    }
}