需求示例：

一个HTTP请求参数类型，用于创建会议。
会议可以为音频、或者视频，所以参数类型要么为音频会议的类型、要么为视频会议的类型，剩下的是两种会议共有的参数。

开始的想法：

interface VideoConferenceRoom {
    mode: "VIDEO";
    layout: 1 | 2 | 3 | 4 | 6 | 9;
    bitrate: number;
    framerate: number;
    resolution: "CIF" | "QCIF" | "VGA" | "SVGA" | "XVGA" | "HD720" | "QVGA";
}

interface AudioConferenceRoom {
    mode: "AUDIO";
}

interface CreateConferenceRoomParams 
    extends VideoConferenceRoom,
    AudioConferenceRoom {
    extension_number: string;
    room_pin: string;
    admin_pin: string;
    subject: string;
    language: string;
    custom_options: string;
    outbound_caller_ids: { provider_id: string; caller_id: string }[];
}
error: Interface 'CreateConferenceRoomParams' cannot simultaneously extend types 'VideoConferenceRoom' and 'AudioConferenceRoom'.
  Named property 'mode' of types 'VideoConferenceRoom' and 'AudioConferenceRoom' are not identical.

失败，因为interface的多重继承需要被继承的接口完全一致，而此处的两个接口的mode字段并不兼容。

那么我们需要一个Unit types的中间类型：

......省略......

// 新增
type ConferenceRoom = AudioConferenceRoom | VideoConferenceRoom;

interface CreateConferenceRoomParams
    extends ConferenceRoom {
    extension_number: string;
    room_pin: string;
    admin_pin: string;
    subject: string;
    language: string;
    custom_options: string;
    outbound_caller_ids: { provider_id: string; caller_id: string }[];
}
error: An interface can only extend an object type or intersection of object types with statically known members.

失败，接口只能继承一个静态类型，而联合类型是不确定的。那么最后的：

interface VideoConference {
  mode: "VIDEO";
  layout: 1 | 2 | 3 | 4 | 6 | 9;
  bitrate: number;
  framerate: number;
  resolution: "CIF" | "QCIF" | "VGA" | "SVGA" | "XVGA" | "HD720" | "QVGA";
}

interface AudioConference {
  mode: "AUDIO";
}

type ConferenceMedia = VideoConference | AudioConference;

export type CreateConferenceRoomParams = ConferenceMedia & {
  extension_number: string;
  room_pin: string;
  admin_pin: string;
  subject: string;
  language: string;
  custom_options: string;
  outbound_caller_ids: { provider_id: string; caller_id: string }[];
};

使用类型别名来实现，将两个类型交叉后，得到一个符合需求的类型组合。
顺便类型名称改了一下，变得更符合语义