I’m trying to write a test for a function in my Node.js/Express application using Vitest. The function generateData processes some request data, calls two other functions (getParsedData and getRefactoredData), and sends a JSON response. I want to mock these two side effect functions to isolate the unit test for generateData.

Here’s the generateData function:

const generateData = catchAsync(
  async (req: Request, res: Response, next: NextFunction) => {
    const parsedData = await getParsedData(req.body);
    const refactoredData = getRefactoredData(parsedData);

    res.status(200).json(refactoredData);
  }
);

This is my test:

import { describe, expect, it, vi } from "vitest";
import { createMocks } from "node-mocks-http";
import { Request, Response, NextFunction } from "express";
import {
  generateData,
} from "../../src/handlers/resume.handler";
import * as resumeHandler from "../../src/handlers/resume.handler";

describe("generateData", () => {
  it("should respond with resume data", async () => {
    const mockNext = vi.fn();
    const { req, res } = createMocks<Request, Response>({
      method: "POST",
      body: mockRawData,
    });
    const spyGetParsedData = vi.spyOn(resumeHandler,"getParsedData").mockReturnValue(mockParsedData);

    const spyGetRefactoredData = vi.spyOn(resumeHandler,"getRefactoredData").mockReturnValue(mockRefactoredData);

    await resumeHandler.generateData(req, res, mockNext);

    expect(spyGetParsedData).toHaveBeenCalledWith(mockRawData);
    expect(spyGetRefactoredData).toHaveBeenCalledWith(mockParsedData);
    expect(res._getStatusCode()).toBe(200);
    expect(JSON.parse(res._getData())).toEqual(mockRefactoredData);

    spyGetParsedData.mockRestore();
    spyGetRefactoredData.mockRestore();
  });
});

However, I’m encountering an issue with the test. When I run it, I get the following error:

AssertionError: expected "getParsedData" to be called with arguments: [ Array(1) ]

Received: 

Number of calls: 0

 ❯ test/handlers/resume.handler.test.ts:108:30
    106|     await resumeHandler.generateData(req, res, mockNext);
    107| 
    108|     expect(spyGetParsedData).toHaveBeenCalledWith(mockRawData);
       |                              ^
    109|     expect(spyGetRefactoredData).toHaveBeenCalledWith(mockParsedData);

When I remove the spies and run the test below, it succeeds:

describe("generateData", () => {
  it("should respond with resume data", async () => {
    const mockNext = vi.fn();
    const { req, res } = createMocks<Request, Response>({
      method: "POST",
      body: mockRawData,
    });

    await resumeHandler.generateData(req, res, mockNext);

    expect(res._getStatusCode()).toBe(200);
    expect(JSON.parse(res._getData())).toEqual(mockRefactoredData);
  });
});