How to write a type guard for an individual interface field?

Question

typescript

There is an interface that has a type field and the type of the attach field depends on the type field.

export interface PostAttach {
    type: "video"|"audio",
    attach: AttachVideo|AttachAudio
}

How can I write a type guard that would receive PostAttach as input and determine the type of the attach field? Like this:

export function isAudio(attach: PostAttach): attach.attach is AttachAudio {
    return attach.type === "audio";
}

Answer 1

interface PostAttachVideo {
  type: 'video'
  attach: AttachVideo
}

interface PostAttachAudio {
  type: 'audio'
  attach: AttachAudio
}

type PostAttach = PostAttachAudio | PostAttachVideo

function isAudio(attach: PostAttach): attach is PostAttachAudio {
    return attach.type === 'audio';
}

sandbox

Answer 2

In fact, there is no need for a separate function here. The usual ifis enough: Sandbox
A with your approach:

export interface PostAttach {
    type: "video"|"audio",
    attach: AttachVideo|AttachAudio
}

TS will not see the connection between the fields typeand attach. For him, these are two unrelated fields. It assumes that any combination of "video"|"audio"and is possible, AttachVideo|AttachAudioand the fact that type = 'video'it says nothing about the type of attach.
Therefore, they need to be divided from one universal interface into several special cases.