如何在Jest中Mock mysql/promise的createConnection函数

要隔离测试你的read类，核心是Mock mysql/promise模块中的createConnection函数，以及连接对象的query方法，完全避免真实数据库交互。以下是针对不同场景的具体实现方案：

1. 基础Mock配置 #

首先在测试文件中Mock整个mysql/promise模块，同时处理TypeScript的类型兼容问题：

import { read } from './path-to-your-read-class';
import * as mysql from 'mysql/promise';
import { Connection, ConnectionOptions, RowDataPacket } from 'mysql/promise';

// Mock整个mysql/promise模块
jest.mock('mysql/promise');

// 为Mock的createConnection添加类型断言
const mockCreateConnection = jest.mocked(mysql.createConnection);

2. 场景测试案例 #

场景1：查询成功，返回正常数据 #

Mock连接成功建立，且query方法返回预设的查询结果：

test('read方法应返回查询到的数据', async () => {
  // 预设模拟的查询结果
  const mockResults: RowDataPacket[] = [
    { id: 1, name: 'test-data-1' },
    { id: 2, name: 'test-data-2' }
  ];

  // Mock连接对象的query方法
  const mockQuery = jest.fn().mockResolvedValue([mockResults]);
  // Mock createConnection返回带有query方法的连接对象
  mockCreateConnection.mockResolvedValue({ query: mockQuery } as unknown as Connection);

  // 实例化read类并调用方法
  const testOptions: ConnectionOptions = { host: 'localhost', user: 'test', password: 'test', database: 'test' };
  const reader = new read(testOptions);
  const results = await reader.read('test_table');

  // 断言结果符合预期
  expect(results).toEqual(mockResults);
  // 断言createConnection被正确调用
  expect(mockCreateConnection).toHaveBeenCalledWith(testOptions);
  // 断言query被调用了正确的SQL语句
  expect(mockQuery).toHaveBeenCalledWith('SELECT * FROM test_table');
});

场景2：查询返回空结果 #

只需修改预设的结果为空数组即可：

test('read方法应返回空数组当表中无数据', async () => {
  const mockEmptyResults: RowDataPacket[] = [];
  const mockQuery = jest.fn().mockResolvedValue([mockEmptyResults]);
  mockCreateConnection.mockResolvedValue({ query: mockQuery } as unknown as Connection);

  const reader = new read({} as ConnectionOptions);
  const results = await reader.read('empty_table');

  expect(results).toEqual([]);
  expect(mockQuery).toHaveBeenCalledWith('SELECT * FROM empty_table');
});

场景3：数据库连接失败 #

Mock createConnection抛出错误，测试方法是否正确传递错误：

test('read方法应在连接失败时抛出错误', async () => {
  const mockError = new Error('数据库连接超时');
  mockCreateConnection.mockRejectedValue(mockError);

  const reader = new read({} as ConnectionOptions);
  
  // 断言方法抛出预期错误
  await expect(reader.read('test_table')).rejects.toThrow(mockError);
  expect(mockCreateConnection).toHaveBeenCalled();
});

场景4：查询执行失败 #

Mock连接成功建立，但query方法抛出错误：

test('read方法应在查询失败时抛出错误', async () => {
  const mockQueryError = new Error('SQL语法错误');
  const mockQuery = jest.fn().mockRejectedValue(mockQueryError);
  mockCreateConnection.mockResolvedValue({ query: mockQuery } as unknown as Connection);

  const reader = new read({} as ConnectionOptions);
  
  await expect(reader.read('invalid_table')).rejects.toThrow(mockQueryError);
  expect(mockQuery).toHaveBeenCalledWith('SELECT * FROM invalid_table');
});

注意事项 #

• 每次测试前可以用jest.clearAllMocks()清除Mock的调用记录，避免测试间的干扰
• TypeScript类型断言（as unknown as Connection）是因为我们只Mock了需要的query方法，完整的Connection对象包含更多属性，但测试中无需关心

内容的提问来源于stack exchange，提问作者BoazBitt