从 didReceiveResponse 回调返回 null 是否有效？答案

【问题标题】：Is it valid to return null from a didReceiveResponse callback?从 didReceiveResponse 回调返回 null 是否有效？
【发布时间】：2020-04-25 03:16:54
【问题描述】：

我在扩展 RESTDataSource 的类上有一个 didReceiveResponse 回调，如果响应状态为 404，我将在其中返回 null。由于输入了 RESTDataSource.didReceiveResponse，这似乎是无效的。

  async didReceiveResponse<T>(res: Response, req: Request): Promise<T | null> {
      if (res.status === 404) {
        return null;
      }

      return super.didReceiveResponse(res, req);
  }

这是我在使用 --strict-null-checks 时遇到的 Typescript 错误

TS2416: Property 'didReceiveResponse' in type 'APIDataSource' is not assignable to the same property in base type 'RESTDataSource<ResolverContext>'.
  Type '<T>(res: Response, req: Request) => Promise<T | null>' is not assignable to type '<TResult = any>(response: Response, _request: Request) => Promise<TResult>'.
    Type 'Promise<TResult | null>' is not assignable to type 'Promise<TResult>'.
      Type 'TResult | null' is not assignable to type 'TResult'.
        Type 'null' is not assignable to type 'TResult'.

有没有办法在不禁用编译器或严格的空检查的情况下在返回 null 的同时解决这种输入问题？

【问题讨论】：

标签： typescript apollo apollo-server apollo-datasource-rest

【解决方案1】：

看看RESTDataSource.ts#L100这个apollo-datasource-rest包的源代码。

didReceiveResponse方法的源码为：

protected async didReceiveResponse<TResult = any>(
    response: Response,
    _request: Request,
): Promise<TResult> {
    if (response.ok) {
      return (this.parseBody(response) as any) as Promise<TResult>;
    } else {
      throw await this.errorFromResponse(response);
    }
}

他们使用这样的类型断言：

return (this.parseBody(response) as any) as Promise<TResult>;

所以，如果您确定 null 值正是您想要的，您也可以这样做

例如

SomeDataSource.ts:

import { RESTDataSource } from 'apollo-datasource-rest';
import { Response, Request } from 'apollo-server-env';

class SomeDataSource extends RESTDataSource {
  protected async didReceiveResponse<TResult>(res: Response, req: Request): Promise<TResult> {
    if (res.status === 404) {
      return (null as any) as Promise<TResult>;
    }

    return super.didReceiveResponse<TResult>(res, req);
  }
}

tsconfig.json:

"strictNullChecks": true /* Enable strict null checks. */,

【讨论】：