Testing Basics

10 examples to get you started with Testing - 7 basic and 3 intermediate.

Prerequisites

• Node.js 24.18.0 with built-in node:test and node:assert.
• TypeScript service with createApp() export pattern.
• Optional: npm install -D tsx supertest @types/supertest for HTTP integration tests.

Basic Examples

1. Testing Pyramid for HTTP APIs

Balance fast unit tests with fewer integration and E2E tests.

        / E2E \          few - critical user journeys
       / contract \      consumer-provider agreements
      / integration \    HTTP + DB with Testcontainers
     /    unit tests  \  many - pure logic, handlers mocked

• Most tests should be unit-level (milliseconds each).
• Integration tests prove wiring; E2E proves deployed environment.
• Contract tests matter when multiple services evolve APIs independently.

2. node:test Smoke Test

Zero-dependency test runner built into Node 24.

import assert from "node:assert/strict";
import { describe, it } from "node:test";
 
import { calculateTax } from "../src/billing/tax.js";
 
describe("calculateTax", () => {
  it("applies rate to subtotal", () => {
    assert.equal(calculateTax(100, 0.2), 20);
  });
});

node --import tsx --test test/tax.test.ts

• No Jest needed for pure functions.
• node:assert/strict uses === semantics.

Related: node:test & node:assert - full runner guide

3. Export createApp for HTTP Tests

Separate server factory from listen for Supertest.

// src/app.ts
import express from "express";
 
export function createApp() {
  const app = express();
  app.get("/health", (_req, res) => res.json({ ok: true }));
  return app;
}

• Tests import createApp() without binding a port.
• Production server.ts calls createApp().listen(...).
• Pattern works for Express 5 and Fastify with app.ready().

Related: Supertest & HTTP Integration - in-process HTTP

4. Arrange-Act-Assert Structure

Readable tests document expected behavior.

it("returns 404 for unknown invoice", async () => {
  // Arrange
  const app = createApp();
  // Act
  const res = await request(app).get("/invoices/missing");
  // Assert
  assert.equal(res.status, 404);
});

• One logical assertion focus per test when possible.
• Name tests as behavior: returns 404 when ....
• Avoid testing private functions; test public HTTP or module API.

5. Isolate Tests with beforeEach

Reset mocks and state between cases.

import { beforeEach, describe, it } from "node:test";
 
let cache: Map<string, string>;
 
beforeEach(() => {
  cache = new Map();
});

• Prevents order-dependent failures.
• Use in-memory stores in unit tests; real DB only in integration suite.

6. npm test Script

Single entry for local and CI.

{
  "scripts": {
    "test": "node --import tsx --test",
    "test:unit": "node --import tsx --test test/unit",
    "test:integration": "node --import tsx --test test/integration"
  }
}

• Split suites when integration needs Docker services.
• CI runs both; developers run unit loop during TDD.

7. Test Environment Variables

Use .env.test or inline defaults for test-only config.

process.env.DATABASE_URL ??= "postgres://localhost:5432/billing_test";
process.env.LOG_LEVEL = "silent";

• Never point tests at production databases.
• LOG_LEVEL=silent keeps test output readable.

Intermediate Examples

8. Integration Test with Real Postgres

Testcontainers spins ephemeral DB in CI.

// test/integration/invoices.test.ts - conceptual placement
// See Testcontainers page for full setup

• Run in separate job with Docker available.
• One schema migrate per suite; truncate tables between tests.

Related: Testcontainers - Postgres/Redis in CI

9. Contract Test Between Services

Consumer defines expected provider response shape.

// Consumer pact test - provider must match published contract
// See Contract & Pact page

• Catches breaking API changes before deploy.
• Run on provider CI verifying all consumer pacts.

Related: Contract & Pact Tests - microservice boundaries

10. Load Test Before Launch

Baseline latency and error rate under expected RPS.

k6 run scripts/load/smoke.js

• Not a replacement for unit tests; validates SLO assumptions.
• Run against staging with production-like data volume.

Related: Load Testing with k6/Artillery - baseline RPS

Stack versions: This page was written for Node.js 24.18.0 (Active LTS), npm 10+, TypeScript 5.6+, Express 5, Fastify 5, and NestJS 11.