使用Jest运行单元测试时遇TypeError错误，求解决方法

我给getOnDemandEnvironmentInstances函数写了Jest单元测试，本来只想验证mockClient的send方法是否被调用，但运行测试时抛出TypeError: Cannot read properties of undefined (reading 'reduce')错误，报错行是被测试函数里对Reservations调用reduce的代码。

单元测试代码 #

import { jest } from '@jest/globals';
import { getOnDemandEnvironmentInstances } from './../getOnDemandEnvironmentInstances';
import { MockClient } from './aws-sdk-client-mock';

describe('getOnDemandEnvironmentInstances', () => {
    it('should return instances correctly when there are reservations and instances', async () => {
        const mockClient = new MockClient;
        const mockClientSend = jest.spyOn(mockClient, "send");
        await getOnDemandEnvironmentInstances(mockClient);
        expect(mockClientSend).toHaveBeenCalled();
    });
});

错误信息 #

13 |       const Reservations = data.Reservations;
  14 |
> 15 |       const instanceList = Reservations.reduce((prev, current) => {
     |                                         ^
  16 |           return prev.concat(current.Instances);
  17 |       }, []);

被测试函数代码 #

import { DescribeInstancesCommand } from "@aws-sdk/client-ec2";

export async function getOnDemandEnvironmentInstances(ec2Client) {
    const describeInstancesCommand = new DescribeInstancesCommand({
        Filters: [
          { Name: "tag:Phase", Values: ["OnDemandEnv"] },
          { Name: "tag:CbzStackTag", Values: ["cbzwebtier-test-ondemand"] },
          { Name: "tag:Platform", Values: ["Windows"] },
        ],
      });
  
      const data = await ec2Client.send(describeInstancesCommand);
      const Reservations = data.Reservations;
  
      const instanceList = Reservations.reduce((prev, current) => {
          return prev.concat(current.Instances);
      }, []);

      return instanceList;
}

问题原因 #

你的mockClient没有模拟send方法的返回值，导致调用ec2Client.send后返回的data对象里没有Reservations属性，Reservations变成undefined，调用reduce自然会报错。哪怕你只关心send是否被调用，测试运行时被测试函数还是会执行完整逻辑，必须保证mock返回符合预期的结构，避免代码执行到报错分支。

修改方案 #

1. 修正单元测试代码

让mock的send方法返回包含Reservations的模拟数据，同时保留对send调用的验证：

import { jest } from '@jest/globals';
import { getOnDemandEnvironmentInstances } from './../getOnDemandEnvironmentInstances';
import { MockClient } from './aws-sdk-client-mock';
import { DescribeInstancesCommand } from "@aws-sdk/client-ec2";

describe('getOnDemandEnvironmentInstances', () => {
    it('should return instances correctly when there are reservations and instances', async () => {
        const mockClient = new MockClient;
        // 模拟send方法返回符合结构的AWS响应数据
        jest.spyOn(mockClient, "send").mockResolvedValue({
            Reservations: [
                { Instances: [{ InstanceId: 'i-12345' }] },
                { Instances: [{ InstanceId: 'i-67890' }] }
            ]
        });

        await getOnDemandEnvironmentInstances(mockClient);
        
        // 验证send被调用，且参数是正确的DescribeInstancesCommand实例
        expect(mockClient.send).toHaveBeenCalledWith(expect.any(DescribeInstancesCommand));
    });
});

2. 优化被测试函数（可选但推荐）

为了避免生产环境中因为AWS返回数据异常导致报错，给Reservations和Instances设置默认值：

import { DescribeInstancesCommand } from "@aws-sdk/client-ec2";

export async function getOnDemandEnvironmentInstances(ec2Client) {
    const describeInstancesCommand = new DescribeInstancesCommand({
        Filters: [
          { Name: "tag:Phase", Values: ["OnDemandEnv"] },
          { Name: "tag:CbzStackTag", Values: ["cbzwebtier-test-ondemand"] },
          { Name: "tag:Platform", Values: ["Windows"] },
        ],
      });
  
      const data = await ec2Client.send(describeInstancesCommand);
      // 给Reservations设置默认空数组，避免undefined调用reduce
      const Reservations = data.Reservations || [];
  
      const instanceList = Reservations.reduce((prev, current) => {
          // 给Instances也加默认值，防止单条Reservation没有Instances的情况
          return prev.concat(current.Instances || []);
      }, []);

      return instanceList;
}

内容的提问来源于stack exchange，提问作者Madhuri Verma