如何使用 jest 和 TypeScript 模拟第三方 Nodejs 模块函数？

我正在尝试模拟第三方节点模块中的函数，特别是fs.readFileSync()功能，使用玩笑。那里有很多示例，但我还没有找到使用 TypeScript 的示例。我有一个简单的，希望是最小的例子，位于github https://github.com/asmallfurrywombat/jest_typescript_mock_test。对于熟悉 jest 的人来说，这可能是一个简单的问题。

这里有几种不同的方式来嘲笑类似的东西fs.readFileSync():

模拟函数

要模拟函数，请使用jest.spyOn() https://jestjs.io/docs/en/jest-object#jestspyonobject-methodname与类似功能相结合mockImplementation() https://jestjs.io/docs/en/mock-function-api#mockfnmockimplementationfn:

import { returnNameInJsonFile } from './index';
import * as fs from 'fs';

describe('index', () => {

  it('returnNameInJsonFile', () => {
    const mock = jest.spyOn(fs, 'readFileSync');  // spy on fs.readFileSync()
    mock.mockImplementation(() => JSON.stringify({ name: 'myname' }));  // replace the implementation

    const name: string = returnNameInJsonFile('test.json');
    expect(name).toBe('myname');

    mock.mockRestore();  // restore fs.readFileSync()
  });

});

使用工厂模拟模块

Pass a 模块工厂到jest.mock() https://jestjs.io/docs/en/jest-object#jestmockmodulename-factory-options:

import { returnNameInJsonFile } from './index';

jest.mock('fs', () => {
  const MOCK_FILE_INFO = { 'test.json': JSON.stringify({ name: 'myname' }) };
  return {
    readFileSync: (fpath, opts) => {
      if (fpath in MOCK_FILE_INFO) {
        return MOCK_FILE_INFO[fpath]
      }
      throw 'unexpected fpath'
    }
  }
});

describe('index', () => {
  it('returnNameInJsonFile', () => {
    const name: string = returnNameInJsonFile('test.json');
    expect(name).toBe('myname'); // 1.0.0 is installed and 2.0.0 is available
  });
});

自动模拟模块

Create 模块的模拟 https://jestjs.io/docs/en/manual-mocks.

Jest将自动使用模拟，除非它是核心 Node 模块（例如fs） 在这种情况下calling jest.mock()是必须的 https://jestjs.io/docs/en/manual-mocks#mocking-node-modules.

__mocks__/fs.ts：

const fs = jest.genMockFromModule('fs');

let mockFiles: object = {};

function __setMockFiles (newMockFiles: object) {
  mockFiles = newMockFiles;
}

function readFileSync(filePath: string) {
  return mockFiles[filePath] || '';
}

// If anyone knows how to avoid the type assertion feel free to edit this answer
(fs as any).__setMockFiles = __setMockFiles;
(fs as any).readFileSync = readFileSync;

module.exports = fs;

索引.test.ts：

import { returnNameInJsonFile } from './index';

jest.mock('fs');  // Required since fs is a core Node module

describe('index', () => {

  const MOCK_FILE_INFO = { 'test.json': JSON.stringify({ name: 'myname' }) };

  beforeEach(() => {
    require('fs').__setMockFiles(MOCK_FILE_INFO);
  });

  it('returnNameInJsonFile', () => {
    const name: string = returnNameInJsonFile('test.json');
    expect(name).toBe('myname'); // 1.0.0 is installed and 2.0.0 is available
  });
});