Typescript 为什么在TypScript中将类型守护的返回类型写成This is(

我正在阅读手册的Classes部分，但有一个差异让我感到困惑.

在以下示例中:

1. 在实施例1中，他们首次显式地编写了返回类型this is Networked & this.这部分我能理解.

2. 然后，在实施例2中，他们只做了this is { value: T }，但if块中box的类型是Box<string> & { value: string; }.

我不知道为什么第二种情况有效.根据第一个例子，我预计应该是this is { value: T } & this.这是否意味着我总是可以省略显式的& this？

实施例1

class FileSystemObject {
  isFile(): this is FileRep {
    return this instanceof FileRep;
  }
  isDirectory(): this is Directory {
    return this instanceof Directory;
  }
  isNetworked(): this is Networked & this { // the return type here.
    return this.networked;
  }
  constructor(public path: string, private networked: boolean) {}
}
 
class FileRep extends FileSystemObject {
  constructor(path: string, public content: string) {
    super(path, false);
  }
}
 
class Directory extends FileSystemObject {
  children: FileSystemObject[];
}
 
interface Networked { // this interface that matters.
  host: string;
}
 
const fso: FileSystemObject = new FileRep("foo/bar.txt", "foo");
 
if (fso.isFile()) {
  fso.content;
} else if (fso.isDirectory()) {
  fso.children;
} else if (fso.isNetworked()) {
  fso.host; // const fso: Networked & FileSystemObject
}

实施例2

class Box<T> {
  value?: T;
 
  hasValue(): this is { value: T } { // here, there is no `& this`.
    return this.value !== undefined;
  }
}
 
const box = new Box<string>();
box.value = "Gameboy";
 
if (box.hasValue()) {
  box.value; // notice the type of box here.
}