开玩笑模拟fs功能 [英] Mock fs function with jest

问题描述

首先，我是es6和jest的新手.

我有一个用于实例化winston的Logger类，我想对其进行测试.

I have a Logger class for instantiate winston and I would like to test it.

这是我的代码:

const winston = require('winston');
const fs = require('fs');
const path = require('path');
const config = require('../config.json');

class Logger {
  constructor() {
    Logger.createLogDir(Logger.logDir);
    this.logger = winston.createLogger({
      level: 'info',
      format: winston.format.json(),
      transports: [
        new (winston.transports.Console)({
          format: winston.format.combine(
            winston.format.colorize({ all: true }),
            winston.format.simple(),
          ),
        }),
        new (winston.transports.File)({
          filename: path.join(Logger.logDir, '/error.log'),
          level: 'error',
        }),
        new (winston.transports.File)({
          filename: path.join(Logger.logDir, '/info.log'),
          level: 'info',
        }),
        new (winston.transports.File)({
          filename: path.join(Logger.logDir, '/combined.log'),
        }),
      ],
    });
  }

  static get logDir() {
    return (config.logDir == null) ? 'log' : config.logDir;
  }

  static createLogDir(logDir) {
    if (!fs.existsSync(logDir)) {
      // Create the directory if it does not exist
      fs.mkdirSync(logDir);
    }
  }
}

exports.logger = new Logger().logger;
export default new Logger();

我想测试我的功能createLogDir(). 我的头，我认为测试fs.existsSync的状态是个好主意. 如果fs.existsSync返回false，则必须调用fs.mkdirSync. 因此，我尝试编写一些jest测试:

I would like to test my function createLogDir(). I my head, I think it's a good idea to test the state of fs.existsSync. If fs.existsSync return false, fs.mkdirSync must be called. So I try to write some jest test :

describe('logDir configuration', () => {
  test('default path must be used', () => {
    const logger = require('./logger');
    jest.mock('fs');
    fs.existsSync = jest.fn();
    fs.existsSync.mockReturnValue(false);
    const mkdirSync = jest.spyOn(logger, 'fs.mkdirSync');
    expect(mkdirSync).toHaveBeenCalled();
  });
});

但是，我遇到了一个错误:

However, I've got an error :

  ● logDir configuration › default path must be used

    Cannot spy the fs.mkdirSync property because it is not a function; undefined given instead

      18 |     fs.existsSync = jest.fn();
      19 |     fs.existsSync.mockReturnValue(true);
    > 20 |     const mkdirSync = jest.spyOn(logger, 'fs.mkdirSync');
      21 |     expect(mkdirSync).toHaveBeenCalled();
      22 |   });
      23 | });

      at ModuleMockerClass.spyOn (node_modules/jest-mock/build/index.js:590:15)
      at Object.test (src/logger.test.js:20:28)

可以帮我调试和测试我的功能吗?

Can you help me to debug and test my function please ?

致谢.

推荐答案

存在错误是因为它正在您的logger对象上寻找一个名为fs.mkdirSync的方法，该方法不存在.如果您可以在测试中访问fs模块，则可以像这样监视mkdirSync方法:

The error there is because it is looking for a method called fs.mkdirSync on your logger object, which doesn't exist. If you had access to the fs module in your test then you would spy on the mkdirSync method like this:

jest.spyOn(fs, 'mkdirSync');

但是，我认为您需要采取其他方法.

However, I think you need to take a different approach.

您的createLogDir函数是静态方法-意味着只能在该类上调用该函数，而不能在该类的实例上调用(new Logger()是该类Logger的实例).因此，为了测试该功能，您需要导出该类而不是其实例，即:

Your createLogDir function is a static method - meaning that it can only be called on the class, and not on an instance of that class (new Logger() is an instance of the class Logger). Therefore, in order to test that function you need to export the class and not an instance of it, i.e.:

module.exports = Logger;

然后您可以进行以下测试:

Then you could have the following tests:

const Logger = require('./logger');
const fs = require('fs');

jest.mock('fs') // this auto mocks all methods on fs - so you can treat fs.existsSync and fs.mkdirSync like you would jest.fn()

it('should create a new log directory if one doesn\'t already exist', () => {
    // set up existsSync to meet the `if` condition
    fs.existsSync.mockReturnValue(false);

    // call the function that you want to test
    Logger.createLogDir('test-path');

    // make your assertion
    expect(fs.mkdirSync).toHaveBeenCalled();
});

it('should NOT create a new log directory if one already exists', () => {
    // set up existsSync to FAIL the `if` condition
    fs.existsSync.mockReturnValue(true);

    Logger.createLogDir('test-path');

    expect(fs.mkdirSync).not.toHaveBeenCalled();
});

注意:看来您正在混合使用CommonJS和es6模块语法(export default是es6)-我会尝试坚持使用另一种方法

Note: it looks like you're mixing CommonJS and es6 module syntax (export default is es6) - I would try to stick to one or the other

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