Merging custom type with one from external library in Typescript

Question

I have a type in an external library that looks like this:

export declare type CreateProductInput = {
    title: string;
}

I want this type to have another attribute so tried to add to it in my own index.d.ts file:

I have tried the following:

export declare type CreateProductInput = {
    subtitle: string;
}

But that did not work. I also tried the following:

declare module '@path/to/library/type/file' {
    declare interface CreateProductInput {
        subtitle: string;
    }
}

However, doing this overwrites the type completely and I no longer have access to the title field.

Can one merge types like this? Basically, I want to modify the original type by adding another property to it.

Answer 1

If you really want to stick with type then you should be able to do this even on an imported type:

type a = {
    title: string
}

type b = a & {
    subtitle: string
}

const c: b = {
    title: "title",
    subtitle: "subtitle"
}

console.log(c)

The other option would be to use an interface:

type d = {
    title: string
}

interface e extends d  {
    subtitle: string
}

const f: e = {
    title: "title",
    subtitle: "subtitle"
}

console.log(f)

Check out both in the playground. What you want to do with overwriting the type the way you're trying to do so isn't possible and is poor form. Create a new type/interface that has what you need.