express-mocks-testing

name	express-mocks-testing
description	Unit testing pattern for Express 5 controllers using node-mocks-http and @prisma/client mocks. Trigger: When writing unit tests for backend controllers, testing Express routes, or avoiding Supertest/Server instantiation.
license	Apache-2.0
metadata	{"author":"gentleman-programming","version":"1.0"}

When to Use

• When unit testing Express 5 controllers in isolation without starting the server.
• When you want to completely decouple from a real database.
• When testing specific logic, role filters, or validation branches inside a controller.
• Do NOT use this for End-to-End (E2E) integration tests that genuinely require testing the full middleware chain (CORS, body-parser, global error handlers). For E2E, use Supertest.

Critical Patterns

• NEVER instantiate the full Server: Do not import or instantiate new Server() or app from server.ts. It causes slow tests and port conflicts.
• Global Prisma Mock: Always use jest.mock("@prisma/client", () => { ... }) to intercept Prisma Client instantiations before the controller executes.
• Isolate Request/Response: Use createRequest and createResponse from node-mocks-http to simulate Express objects.
• Call Controller Directly: Invoke the controller method directly (e.g., await controller.sendMessage(req, res)).
• Clear Mocks: Always run jest.clearAllMocks() in the beforeEach hook.
• Assertions: Use res._getJSONData() to verify JSON responses and res.statusCode to check HTTP statuses.

Code Examples

Basic Controller Test Setup

import { createRequest, createResponse } from "node-mocks-http";
import { MyController } from "../my.controller";
import { PrismaClient } from "@prisma/client";

// 1. MOCK PRISMA GLOBALLY
jest.mock("@prisma/client", () => {
  const mPrismaClient = {
    user: {
      findUnique: jest.fn(),
      create: jest.fn(),
    },
  };
  return { PrismaClient: jest.fn(() => mPrismaClient) };
});

const prismaMock = new PrismaClient() as jest.Mocked<any>;

describe("MyController", () => {
  let controller: MyController;

  beforeAll(() => {
    controller = new MyController();
  });

  beforeEach(() => {
    jest.clearAllMocks();
  });

  describe("getUser", () => {
    it("should return 200 and the user data", async () => {
      // Arrange
      const req = createRequest({
        method: "GET",
        url: "/api/users/1",
        params: { id: "1" },
        user: { id_persona: "admin-123" } // If testing protected routes
      });
      const res = createResponse();

      prismaMock.user.findUnique.mockResolvedValue({ id: "1", name: "John" });

      // Act
      await controller.getUser(req, res);

      // Assert
      expect(res.statusCode).toBe(200);
      expect(res._getJSONData().name).toBe("John");
      expect(prismaMock.user.findUnique).toHaveBeenCalledWith({
        where: { id: "1" }
      });
    });
  });
});

Commands

# Run backend tests
pnpm test

# Run specific test file
pnpm jest path/to/your/controller.test.ts