How to build and test a Node.js REST API with Jest and Supertest

What you will build

You will build a small REST API with Express and test its HTTP endpoints. Jest is a popular JavaScript test runner with assertions and mocking. Supertest sends real HTTP requests to your app in memory, so you can assert on status codes and response bodies without starting a network server. The key practice is separating the app from the server so tests can import the app directly.

Prerequisites

• Node.js 18 or later
• Basic JavaScript and HTTP knowledge

Steps

1. Create the project

mkdir todo-api && cd todo-api
npm init -y
npm install express

2. Build a testable Express app

Export the app separately from the code that calls listen, so tests import the app without binding a port.

// app.js
const express = require("express");
const app = express();
app.use(express.json());

let todos = [];
app.get("/todos", (req, res) => res.json(todos));
app.post("/todos", (req, res) => {
  const todo = { id: todos.length + 1, title: req.body.title };
  todos.push(todo);
  res.status(201).json(todo);
});
module.exports = app;

3. Install Jest and Supertest

npm install -D jest supertest

4. Write endpoint tests

// app.test.js
const request = require("supertest");
const app = require("./app");

test("creates and lists todos", async () => {
  const created = await request(app).post("/todos").send({ title: "buy milk" });
  expect(created.status).toBe(201);
  expect(created.body.title).toBe("buy milk");

  const list = await request(app).get("/todos");
  expect(list.body).toHaveLength(1);
});

5. Add setup and teardown

Use beforeEach to reset shared state so tests do not interfere with each other.

beforeEach(() => { /* reset database or in-memory store */ });

6. Run tests and coverage

Add a script and run it.

{ "scripts": { "test": "jest --coverage" } }

npm test

Verification

Run npm test and confirm both the create and list assertions pass. Add a test that posts without a title and assert on the resulting behavior. Break the route to return the wrong status and confirm the test catches it.

Next Steps

Mock external services, test error paths and validation, use a separate test database, and run the suite in continuous integration.