Merging custom type with one from external library in Typescript-CodePudding

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.

CodePudding user response：

Unfortunately, a type alias cannot be merged. That is one of the differences with an interface declaration in TypeScript.

But you can create a new interface that extends a type, although this does not seem what you want here:

type CreateProductInput = {
    title: string;
}

interface CreateProductInput2 extends CreateProductInput  {
    subtitle: string;
}

declare const test: CreateProductInput2;

test.title; // Okay
test.subtitle;

Playground Link

For more details, see e.g. Interfaces vs Types in TypeScript

CodePudding user response：

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.