How do you test Node.js applications with Jest?

Direct answer

Jest runs unit and integration tests — describe/it, expect matchers, mock modules with jest.mock, test async with async/await; use supertest or inject() for HTTP; separate test DB; run in CI with NODE_ENV=test.

Testing Node services focuses on pure functions, handlers, and repositories — mock external I/O (pg, Redis, fetch) for fast unit tests; integration tests hit real Docker Postgres on port 5433.

Layer	What to test	Tool
Unit	Service logic, mappers	Jest + mocks
HTTP	Route status/body	Fastify inject() or supertest
Integration	SQL repositories	Test DB + transactions rollback
E2E	Full flows	Separate suite; slower CI job

jest.mock('pg') — stub pool.query return values.
beforeEach/afterAll — reset mocks; close server and pool.
Coverage — CI gate on critical modules; not 100% everywhere.

jest-test.mjs

import { describe, it, expect, jest } from "@jest/globals";

describe("interviewsService", () => {
  it("returns topic page for valid slug", async () => {
    jest.spyOn(repo, "findTopicBySlug").mockResolvedValue({ slug: "nodejs-interview-questions" });

    const result = await service.getTopicPage("nodejs-interview-questions");
    expect(result.slug).toBe("nodejs-interview-questions");
  });
});

// HTTP: const res = await app.inject({ method: "GET", url: "/health" });