Sophisticated TypeScript parameter limitations

I have a question regarding the possibility of achieving the following in TypeScript: What I want to accomplish:

  1. If the type is set to inbox, then the obj should have the type interface IInbox.
  2. If the type is set to sent, then the obj should have the type interface ISent.

interface IInbox {

}

interface ISent {

}

class MailClient {
    delete(type: "inbox" | "sent", obj: IInbox | ISent) {

    }
}

let client = new MailClient();
client.delete('inbox', <ISent>{}); // This should result in a compile error

Answer №1

You have the option to specify multiple signatures:

class MailClient {
    delete(type: "inbox", obj: IInbox);
    delete(type: "sent", obj: ISent)
    delete(type: "inbox" | "sent", obj: IInbox | ISent) {}
}

However, even with different signatures, there may not be a compilation error if your interfaces are identical.
Due to TypeScript's utilization of duck typing, an empty object ({}) fulfills the type requirements.
If you make a distinction between them:

interface IInbox {
    a: string;
}

interface ISent {
    b: string;
}

Then you will encounter an error:

client.delete('inbox', {} as ISent); // Argument of type '"inbox"' is not assignable to parameter of type '"sent"'

(code in playground)

Answer №2

From my understanding, accomplishing this constraint may not be possible. However, a simple workaround can be implemented as shown below:

class MailClient {
    remove(type: "inbox" | "sent", inboxItem?: IInbox, sentItem?: ISent) {
        if ((type === "inbox" && typeof sentItem !== "undefined") ||
                 type === "send" && typeof inboxItem !== "undefined") {
            throw new "Invalid parameter";    
        }
        let item = (type === "inbox") ? inboxItem : sentItem;
    }
}

This solution should effectively handle your requirements in a clear and understandable manner.

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