使用Sinon进行Stub时测试用例无法正常运行

问题现象 #

测试bar函数时，使用Sinon为createFile创建Stub，但运行npm test时出现如下错误：

1) index.js
       stub function
         should stub log:
     AssertionError: expected greet to have been called exactly once, but it was called 0 times
      at Context.<anonymous> (test/lib/index.test.js:26:31)
      at process.processImmediate (node:internal/timers:478:21)

相关代码 #

index.js #

async function bar(fileName) {
  await createFile(fileName);
}

function createFile(fileName) {
  // public
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve("done");
    }, 100);
  });
}

module.exports = {
  bar,
  createFile,
};

index.test.js #

const chai = require('chai');
const sinon = require('sinon');
const sinonChai = require('sinon-chai');
const expect = chai.expect;

chai.use(sinonChai);

const demo = require("./index.js");

describe("index.js", () => {
    context("stub function", () => {
        it("should stub createFile and callDB", async () => {
            // Stub createFile function in the scope where it's being imported (i.e., demo module)
            let stub = sinon.stub(demo, "createFile").resolves("create_stub");
            await demo.bar('test.txt');
            expect(stub).to.be.calledOnce;
            stub.restore();
        });
    });
});

package.json 依赖 #

"dependencies": {
    "chai": "^4.4.0",
    "cross-env": "^7.0.3",
    "rewire": "^7.0.0",
    "sinon": "^17.0.1",
    "sinon-chai": "^3.7.0"
  }

核心原因 #

bar函数内部直接调用的是模块内的本地createFile函数引用，而Sinon Stub的是模块导出的createFile函数。两者不是同一个引用，所以实际执行bar时不会触发Stub。

解决方案 #

方案一：修改模块内部调用为导出的函数 #

调整bar函数，让它调用模块导出的createFile：

async function bar(fileName) {
  // 改为调用module.exports上的createFile
  await module.exports.createFile(fileName);
}

function createFile(fileName) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve("done");
    }, 100);
  });
}

module.exports = {
  bar,
  createFile,
};

方案二：使用rewire修改模块内部私有变量 #

利用rewire模块直接替换模块内部的createFile函数：

const chai = require('chai');
const sinon = require('sinon');
const sinonChai = require('sinon-chai');
const rewire = require('rewire');
const expect = chai.expect;

chai.use(sinonChai);

// 用rewire加载目标模块
const demo = rewire("./index.js");

describe("index.js", () => {
    context("stub function", () => {
        it("should stub createFile and callDB", async () => {
            // 创建Stub并替换模块内部的createFile
            const createFileStub = sinon.stub().resolves("create_stub");
            demo.__set__('createFile', createFileStub);
            
            await demo.bar('test.txt');
            expect(createFileStub).to.be.calledOnce;
            
            // 恢复原函数
            demo.__restore__();
        });
    });
});

方案三：重构代码，抽离独立模块 #

将createFile抽离到单独的工具模块，这样测试时可以直接Stub该模块的导出：

1. 新建utils.js：

function createFile(fileName) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve("done");
    }, 100);
  });
}

module.exports = { createFile };

2. 修改index.js：

const { createFile } = require('./utils');

async function bar(fileName) {
  await createFile(fileName);
}

module.exports = { bar };

3. 修改测试文件：

const chai = require('chai');
const sinon = require('sinon');
const sinonChai = require('sinon-chai');
const expect = chai.expect;
const utils = require('./utils');
const demo = require("./index.js");

chai.use(sinonChai);

describe("index.js", () => {
    context("stub function", () => {
        it("should stub createFile and callDB", async () => {
            const createFileStub = sinon.stub(utils, "createFile").resolves("create_stub");
            await demo.bar('test.txt');
            expect(createFileStub).to.be.calledOnce;
            createFileStub.restore();
        });
    });
});

内容的提问来源于stack exchange，提问作者rafl952