The concept of Typescript involves taking a particular type and generating a union type within a generic interface

Picture a straightforward CollectionStore that contains methods for creating and updating a record. The create() method takes in a set of attributes and returns the same set with an added id property. On the other hand, the update method requires the set to have an already defined id property.

In Typescript, how can I indicate that the create() function accepts a type T and returns T & {id: string}?

The expected pattern might look something like this:

interface CollectionStore<T> {
    updateRecord(T & {id: string}): void;
    createRecord(T): T & {id: string};
}

However, the above code snippet is not valid. Assistance would be greatly appreciated! =)

Answer №1

Your approach to using the union type is correct, but you forgot to assign names to the function parameters which resulted in an error. Here's the corrected version:

interface CollectionStore<T> {
    updateRecord(record: T & { id: string }): void;
    createRecord(record: T): T & { id: string };
}

After that:

interface MyRecord {
    key: string;
}

let a: CollectionStore<MyRecord> = ...;

a.updateRecord({ key: "key", id: "id" });
a.createRecord({ key: "key" });

(code in playground)

Alternatively, you can define a base interface for records where the id property is optional:

interface Record {
    id?: string;
}

interface CollectionStore<T extends Record> {
    updateRecord(record: T): void;
    createRecord(record: T): T;
}

However, this approach eliminates the ability to ensure that updateRecord returns an object with an id.

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