Improve transient field usability

[mizdra]

close: #17
follow up: #12

Before

import {
  defineBookFactory,
  lazy,
  defineAuthorFactory,
  defineAuthorFactoryWithTransientFields,
} from './index.js';

const BookFactory = defineBookFactory({
  defaultFields: {
    id: lazy(({ seq }) => `Book-${seq}`),
    title: lazy(({ seq }) => `ゆゆ式 ${seq}巻`),
    author: undefined,
  },
});
const AuthorFactory = defineAuthorFactoryWithTransientFields(
  {
    bookCount: 0,
  },
  {
    defaultFields: {
      id: lazy(({ seq }) => `Author-${seq}`),
      name: '三上小又',
      books: lazy(async ({ get }) => {
        const bookCount = (await get('bookCount')) ?? 0;
        // eslint-disable-next-line max-nested-callbacks
        return Promise.all(Array.from({ length: bookCount }, async () => BookFactory.build()));
      }),
    },
  },
);
const author1 = await AuthorFactory.build();
expect(author1).toStrictEqual({
  id: 'Author-0',
  name: '三上小又',
  books: [],
});
assertType<{
  id: string;
  name: string;
  books: { id: string; title: string; author: undefined }[];
}>(author1);

After

import {
  defineBookFactory,
  lazy,
  defineAuthorFactory,
  AuthorFactoryDefineOptions,
  AuthorFactoryInterface,
} from './index.js';

// Define factory with transient fields
type AuthorTransientFields = {
  bookCount: number;
};
function defineAuthorFactoryWithTransientFields<
  Options extends AuthorFactoryDefineOptions<AuthorTransientFields>,
>(options: Options): AuthorFactoryInterface<AuthorTransientFields, Options> {
  return defineAuthorFactory(options);
}

const BookFactory = defineBookFactory({
  defaultFields: {
    id: lazy(({ seq }) => `Book-${seq}`),
    title: lazy(({ seq }) => `ゆゆ式 ${seq}巻`),
    author: undefined,
  },
});
const AuthorFactory = defineAuthorFactoryWithTransientFields({
  defaultFields: {
    id: lazy(({ seq }) => `Author-${seq}`),
    name: '三上小又',
    books: lazy(async ({ get }) => {
      const bookCount = (await get('bookCount')) ?? 0;
      // eslint-disable-next-line max-nested-callbacks
      return Promise.all(Array.from({ length: bookCount }, async () => BookFactory.build()));
    }),
    bookCount: 0,
  },
});
const author1 = await AuthorFactory.build();
expect(author1).toStrictEqual({
  id: 'Author-0',
  name: '三上小又',
  books: [],
});
assertType<{
  id: string;
  name: string;
  books: { id: string; title: string; author: undefined }[];
}>(author1);