Add an extend method

[LandonSchropp]

Is your feature request related to a problem? Please describe.

I'm working on my personal website that presents a few different types of content:

• Article
• TodayILearned
• Note

In order to avoid duplicating functionality, both in my code and in my Zod schema, I've added a base Content type that the other types extend.

Extending factories for these is a bit of a pain. For example, here's what my content factory currently looks like:

export class ContentFactory extends Factory<Content> {
  idea() {
    return this.params({ status: IDEA_STATUS });
  }

  draft() {
    return this.params({ status: DRAFT_STATUS });
  }

  published() {
    return this.params({ status: PUBLISHED_STATUS });
  }

  willNotPublish() {
    return this.params({ status: WILL_NOT_PUBLISH_STATUS });
  }
}

export const ContentFactory = ContentFactory.define<Content>(
  ({ sequence }) => ({
    title: `Test Content ${sequence}`,
    date: "2024-01-15",
    status: PUBLISHED_STATUS,
    markdown: "This is test markdown content.",
    filePath: `/test/content-${sequence}.md`,
  }),
);

I haven't found a simple method of extending this factory to inherit the default properties in define and include the traits in a child factory (as well as adding others). I tried something like this:

Describe the solution you'd like

My proposal is similar to #169. I'd really like an extend method capable of extending an existing factory with a new type.

const articleFactory = conentFactory.extend<Article>((sequence) => {
  url: `http://example.com/${sequence}`
  description: `An external article ${sequence}.`,
})

This could then work with the trait method proposed in #169, which would inherit the traits provided in the parent factory.

Describe alternatives you've considered

• I could use factory builder functions. This is doable, but not quite as clean as simply being able to extend the factory.
• I could try to find a way to inherit parent factory classes. However, I didn't see a simple way of extending the generator function in the factory, and this felt overly complicated.
• I could live without reusing trait functions from the parent class, which is probably what I'm going to do.

Additional context

Thanks!