typescript - Jest : Test recursive call inside Promise

Question

我在测试以下类(class)时遇到了一些问题。

interface Connector {
   connect: () => Promise<void>;
}

class Unit {
   private connector: Connector;

   constructor(connector: Connector) {
      this.connector = connector;
   }

   public attemptConnect(iteration: number, max: number): void {
      console.log("attempt" + iteration);

      this.connector.connect()
         .then(() => {
            console.log("connected");
         })
         .catch((error) => {
            if (iteration < max) {
               this.attemptConnect(iteration + 1, max);
            }
         });
   }
}

我想测试 Connector.connect() 函数被调用的次数是否正确。我正在尝试通过以下方式开 Jest :

describe("Unit", () => {
   it("fails to record more than one recursion", async () => {
      const connector: Connector = {
         connect: jest.fn().mockRejectedValue(new Error("foo")),
      };
      const unit: Unit = new Unit(connector);

      await unit.attemptConnect(1, 4);

      expect((connector.connect as jest.Mock).mock.calls.length).toBe(4);
   });
});

不幸的是 expect() 调用失败，说

Error: expect(received).toBe(expected) // Object.is equality

Expected: 4
Received: 1

如果我使用调试器并观察的值

(connector.connect as jest.Mock).mock.calls.length

我可以看到通话记录正确。只有当测试完成时，数字才错误。

感谢您的帮助!

Answer 1

解决方案一

在 attemptConnect() 中返回 promise :

public attemptConnect(iteration: number, max: number): Promise<void> {
  console.log("attempt" + iteration);

  return this.connector.connect()
    .then(() => {
      console.log("connected");
    })
    .catch((error) => {
      if (iteration < max) {
        return this.attemptConnect(iteration + 1, max);
      }
    });
}

方案二

await 测试中所需的事件循环周期数:

describe("Unit", () => {
  it("fails to record more than one recursion", async () => {
    const connector: Connector = {
      connect: jest.fn().mockRejectedValue(new Error("foo")),
    };
    const unit: Unit = new Unit(connector);

    unit.attemptConnect(1, 4);

    // await enough event loop cycles for all the callbacks queued
    // by then() and catch() to run, in this case 5:
    await Promise.resolve().then().then().then().then();

    expect((connector.connect as jest.Mock).mock.calls.length).toBe(4);
  });
});

详情

测试等待attemptConnect()，但由于它没有返回任何东西，所以没有什么可以await，同步测试继续执行。在 then() 和 catch() 排队的回调有机会运行之前，expect() 运行并失败。