Typescript Factories

factories-library.ts

import { times, upperFirst } from 'lodash';

type GeneratorFnOptions = {
  sequence: number;
};
type GeneratorFn<T> = (opts: GeneratorFnOptions) => T;
type GeneratorsMap = {
  [key: string]: GeneratorFn<any>;
};

class FactoryClass {
  generators: GeneratorsMap = {};

  define<T>(name: string, generator: GeneratorFn<T>) {
    this.generators[name] = generator;
  }

  build<T>(name: string, options: Partial<T>): T {
    return {
      ...this.generators[name]({ sequence: nextId() }),
      ...options,
    };
  }

  buildList<T>(number: number, name: string, options: Partial<T>): T[] {
    return times(number, () => {
      return this.build<T>(name, options);
    });
  }
}

export const Factory = new FactoryClass();

factories.ts

import { snakeCase } from 'lodash';
import Factory from 'factories-library';
import { User } from 'lib/types';

// User of the library define factories like this:
Factory.define<User>('user', ({ sequence }) => {
  const name = 'Bill Bob';
  
  return {
    id: sequence,
    fullName: name,
    username: snakeCase(name),
    createdAt: Date.now(),
  };
});

my-test.ts

// Factories are used like this:
// TODO, need mechanism to autoload all factories before tests
import Factory from 'factories-library';
import { User } from 'lib/types';

describe('factories', () => {
  test('work', () => {
    const user = Factory.build<User>('user', { username: 'susan' });
    
    // alternatively, trying to simplify usage to something like (no luck yet with typings):
    // const user = Factory.createUser({ username: 'susan' })
      
    expect(user.fullName).toEqual('Bill Bob');
    expect(user.username).toEqual('susan');
  });
});