Builder Pattern (Typescript)

requestBuilder.ts

type Methods = 'get' | 'post';

interface Req {
  data: object;
  method: Methods;
  url: string;
  foo?: number
}

class Req implements Req {
  constructor(req: Req) {
    Object.assign(this, req);
  }
}

class ReqBuilder implements Partial<Req> {
  data?: object;
  method?: Methods;
  url?: string;
  foo?: number

  setData(data: object): this & Pick<Req, 'data'> {
    return Object.assign(this, { data })
  }

  setMethod(method: Methods): this & Pick<Req, 'method'> {
    return Object.assign(this, { method })
  }

  setUrl(url: string): this & Pick<Req, 'url'> {
    return Object.assign(this, { url })
  }

  setFoo(foo: number): this & Required<Pick<Req, 'foo'>> {
    return Object.assign(this, { foo })
  }

  build(this: Req) {
    return new Req(this)
  }
}

// works
new ReqBuilder()
  .setData({})
  .setMethod('post')
  .setUrl('bar')
  .build()

// works with optional setFoo
new ReqBuilder()
  .setData({})
  .setMethod('post')
  .setUrl('bar')
  .setFoo(42)
  .build()

// can't call .build() unless required setters are called first
new ReqBuilder()
  .setData({})
  .build()