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用Jest/Typescript测试fs库函数

jestjs mocking testing typescript

Steven Scott · 技术社区 · 6 年前

我正在尝试测试一个我已经编写的库函数(它在我的代码中工作),但是无法使用fs的mock进行测试。我有一系列用于操作系统的函数包装在函数中,因此应用程序的不同部分可以使用相同的调用。

下面是一个简短的示例来演示我的问题的基本知识:

import * as fs from 'fs';
export function ReadFileContentsSync(PathAndFileName:string):string {
    if (PathAndFileName === undefined || PathAndFileName === null || PathAndFileName.length === 0) {
        throw new Error('Need a Path and File');
    }
    return fs.readFileSync(PathAndFileName).toString();
}

import { ReadFileContentsSync } from "./read-file-contents-sync";
const fs = require('fs');

describe('Return Mock data to test the function', () => {
    it('should return the test data', () => {
        const TestData:string = 'This is sample Test Data';

// Trying to mock the reading of the file to simply use TestData
        fs.readFileSync = jest.fn();                
        fs.readFileSync.mockReturnValue(TestData);

// Does not need to exist due to mock above     
        const ReadData = ReadFileContentsSync('test-path');
        expect(fs.readFileSync).toHaveBeenCalled();
        expect(ReadData).toBe(TestData);
    });
});

我得到一个异常,即该文件不存在,但我希望没有调用对fs.readFileSync的实际调用,而是使用了jest.fn()mock。

ENOENT: no such file or directory, open 'test-path'

2 回复 | 直到 6 年前

unional 6 年前

既然我提到了functional/OO/和对jest mock的厌恶,我觉得我应该在这里做一些解释。

我不反对 jest.mock() 或任何模仿库(如 sinon ). 但是我发现我自己在大多数情况下都不需要它们,而且在使用它们的时候有一些折衷。

让我首先演示三种不使用mock实现代码的方法。

context 作为第一个论点:

// read-file-contents-sync.ts
import fs from 'fs';
export function ReadFileContentsSync({ fs } = { fs }, PathAndFileName: string): string {
    if (PathAndFileName === undefined || PathAndFileName === null || PathAndFileName.length === 0) {
        throw new Error('Need a Path and File');
    }
    return fs.readFileSync(PathAndFileName).toString();
}

// read-file-contents-sync.spec.ts
import { ReadFileContentsSync } from "./read-file-contents-sync";

describe('Return Mock data to test the function', () => {
    it('should return the test data', () => {
        const TestData:Buffer = new Buffer('This is sample Test Data');

        // Trying to mock the reading of the file to simply use TestData
        const fs = {
            readFileSync: () => TestData
        }

        // Does not need to exist due to mock above     
        const ReadData = ReadFileContentsSync({ fs }, 'test-path');
        expect(ReadData).toBe(TestData.toString());
    });
});

第二种方法是使用OO:

// read-file-contents-sync.ts
import fs from 'fs';
export class FileReader {
    fs = fs
    ReadFileContentsSync(PathAndFileName: string) {
        if (PathAndFileName === undefined || PathAndFileName === null || PathAndFileName.length === 0) {
            throw new Error('Need a Path and File');
        }
        return this.fs.readFileSync(PathAndFileName).toString();
    }
}

// read-file-contents-sync.spec.ts
import { FileReader } from "./read-file-contents-sync";

describe('Return Mock data to test the function', () => {
    it('should return the test data', () => {
        const TestData: Buffer = new Buffer('This is sample Test Data');

        const subject = new FileReader()
        subject.fs = { readFileSync: () => TestData } as any

        // Does not need to exist due to mock above     
        const ReadData = subject.ReadFileContentsSync('test-path');
        expect(ReadData).toBe(TestData.toString());
    });
});

// read-file-contents-sync.ts
import fs from 'fs';
export function ReadFileContentsSync(PathAndFileName: string): string {
    if (PathAndFileName === undefined || PathAndFileName === null || PathAndFileName.length === 0) {
        throw new Error('Need a Path and File');
    }
    return ReadFileContentsSync.fs.readFileSync(PathAndFileName).toString();
}
ReadFileContentsSync.fs = fs

// read-file-contents-sync.spec.ts
import { ReadFileContentsSync } from "./read-file-contents-sync";

describe('Return Mock data to test the function', () => {
    it('should return the test data', () => {
        const TestData: Buffer = new Buffer('This is sample Test Data');

        // Trying to mock the reading of the file to simply use TestData
        ReadFileContentsSync.fs = {
            readFileSync: () => TestData
        } as any

        // Does not need to exist due to mock above     
        const ReadData = ReadFileContentsSync('test-path');
        expect(ReadData).toBe(TestData.toString());
    });
});

前两种方法提供了更大的灵活性和隔离性,因为每个调用/实例都有自己的依赖引用。这意味着一个测试的“模拟”不可能影响另一个测试。

所有这些的底层是依赖关系管理。

依靠模仿库(尤其是像 笑话 )很容易养成忽视这一重要方面的习惯。

有一篇很好的文章我建议大家去看看,那就是Bob叔叔的干净架构: https://8thlight.com/blog/uncle-bob/2012/08/13/the-clean-architecture.html

Steven Scott 6 年前

虽然unional的评论帮助我指出了正确的方向,但fs的导入是在我的代码中完成的 import * as fs from 'fs' . 这似乎就是问题所在。将此处的导入更改为 import fs from 'fs' 这就解决了问题。

因此,代码变为:

import fs from 'fs';
export function ReadFileContentsSync(PathAndFileName:string):string {
    if (PathAndFileName === undefined || PathAndFileName === null || PathAndFileName.length === 0) {
        throw new Error('Need a Path and File');
    }
    return fs.readFileSync(PathAndFileName).toString();
}

jest.mock('fs');
import { ReadFileContentsSync } from "./read-file-contents-sync";

import fs from 'fs';

describe('Return Mock data to test the function', () => {
    it('should return the test data', () => {
        const TestData:Buffer = new Buffer('This is sample Test Data');

// Trying to mock the reading of the file to simply use TestData
        fs.readFileSync = jest.fn();                
        fs.readFileSync.mockReturnValue(TestData);

// Does not need to exist due to mock above     
        const ReadData = ReadFileContentsSync('test-path');
        expect(fs.readFileSync).toHaveBeenCalled();
        expect(ReadData).toBe(TestData.toString());
    });
});