Typescript Interface - Possible to make "one or the other" properties required?

Issue

Possibly an odd question, but I'm curious if it's possible to make an interface where one property or the other is required.

So, for example...

interface Message {
    text: string;
    attachment: Attachment;
    timestamp?: number;
    // ...etc
}

interface Attachment {...}

In the above case, I'd like to make sure that either text or attachment exists.

---

This is how I'm doing it right now. Thought it was a bit verbose (typing botkit for slack).

interface Message {
    type?: string;
    channel?: string;
    user?: string;
    text?: string;
    attachments?: Slack.Attachment[];
    ts?: string;
    team?: string;
    event?: string;
    match?: [string, {index: number}, {input: string}];
}

interface AttachmentMessageNoContext extends Message {
    channel: string;
    attachments: Slack.Attachment[];
}

interface TextMessageNoContext extends Message {
    channel: string;
    text: string;
}

Solution

You can use a union type to do this:

interface MessageBasics {
  timestamp?: number;
  /* more general properties here */
}
interface MessageWithText extends MessageBasics {
  text: string;
}
interface MessageWithAttachment extends MessageBasics {
  attachment: Attachment;
}
type Message = MessageWithText | MessageWithAttachment;

If you want to allow both text and attachment, you would write

type Message = MessageWithText | MessageWithAttachment | (MessageWithText & MessageWithAttachment);

Answered By - Ryan Cavanaugh