如何用 Jest 模拟 fs.promises.writeFile

Question

我正在尝试模拟使用 Jest的promise版本fs.writeFile，并且没有调用被模拟的函数。

要测试的功能（createFile.js)：

const { writeFile } = require("fs").promises;

const createNewFile = async () => {
    await writeFile(`${__dirname}/newFile.txt`, "Test content");
};

module.exports = {
    createNewFile,
};

玩笑测试（createFile.test.js）：

const fs = require("fs").promises;
const { createNewFile } = require("./createFile.js");

it("Calls writeFile", async () => {
    const writeFileSpy = jest.spyOn(fs, "writeFile");

    await createNewFile();
    expect(writeFileSpy).toHaveBeenCalledTimes(1);

    writeFileSpy.mockClear();
});

我知道writeFile实际上正在调用它，因为我运行node -e "require(\"./createFile.js\").createNewFile()"并创建了文件。

依赖版本

• Node.js：14.1.0
• 开玩笑：26.6.3

-- 这是对createFile.test.js文件的另一次尝试--

const fs = require("fs");
const { createNewFile } = require("./createFile.js");

it("Calls writeFile", async () => {
    const writeFileMock = jest.fn();

    jest.mock("fs", () => ({
        promises: {
            writeFile: writeFileMock,
        },
    }));

    await createNewFile();
    expect(writeFileMock).toHaveBeenCalledTimes(1);
});

这会引发以下错误：

ReferenceError: /Users/danlevy/Desktop/test/src/createFile.test.js: The module factory of `jest.mock()` is not allowed to reference any out-of-scope variables.
    Invalid variable access: writeFileMock

Answer 1

由于writeFile在导入时被解构而不是始终被称为fs.promises.writeFile方法，因此它不会受到spyOn.

它应该像任何其他模块一样被嘲笑：

jest.mock("fs", () => ({
  promises: {
    writeFile: jest.fn().mockResolvedValue(),
    readFile: jest.fn().mockResolvedValue(),
  },
}));

const fs = require("fs");

...

await createNewFile();
expect(fs.promises.writeFile).toHaveBeenCalledTimes(1);

模拟fs几乎没有意义，因为未模拟的函数会产生副作用，并可能对测试环境产生负面影响。

Answer 2

在玩笑中模拟“fs/promises”异步函数

这是一个使用 fs.readdir() 的简单示例，但它也适用于任何其他异步 fs/promises 函数。

文件.service.test.js

import fs from "fs/promises";
import FileService from "./files.service";

jest.mock("fs/promises");

describe("FileService", () => {
  var fileService: FileService;

  beforeEach(() => {
    // Create a brand new FileService before running each test
    fileService = new FileService();

    // Reset mocks
    jest.resetAllMocks();
  });

  describe("getJsonFiles", () => {
    it("throws an error if reading the directory fails", async () => {
      // Mock the rejection error
      fs.readdir = jest.fn().mockRejectedValueOnce(new Error("mock error"));

      // Call the function to get the promise
      const promise = fileService.getJsonFiles({ folderPath: "mockPath", logActions: false });

      expect(fs.readdir).toHaveBeenCalled();
      await expect(promise).rejects.toEqual(new Error("mock error"));
    });

    it("returns an array of the .json file name strings in the test directory (and not any other files)", async () => {
      const allPotentialFiles = ["non-json.txt", "test-json-1.json", "test-json-2.json"];
      const onlyJsonFiles = ["test-json-1.json", "test-json-2.json"];

      // Mock readdir to return all potential files from the dir
      fs.readdir = jest.fn().mockResolvedValueOnce(allPotentialFiles);

      // Get the promise
      const promise = fileService.getJsonFiles({ folderPath: "mockPath", logActions: false });

      expect(fs.readdir).toBeCalled();
      await expect(promise).resolves.toEqual(onlyJsonFiles); // function should only return the json files
    });
  });
});

文件.service.ts

import fs from "fs/promises";

export default class FileService {
  constructor() {}

  async getJsonFiles(args: FilesListArgs): Promise<string[]> {
    const { folderPath, logActions } = args;
    try {
      // Get list of all files
      const files = await fs.readdir(folderPath);

      // Filter to only include JSON files
      const jsonFiles = files.filter((file) => {
        return file.includes(".json");
      });

      return jsonFiles;
    } catch (e) {
      throw e;
    }
  }
}