Testing Your API (Jest + Supertest)

Why automate testing?

So far you have tested your API by hand — opening Postman, clicking, eyeballing the result. That works for one endpoint, but the moment you have ten routes and change one, you would have to re-click all of them to be sure you broke nothing. Nobody does that reliably. Automated tests are little programs that call your endpoints and check the answers for you, in seconds, every time you run them. When a test that used to pass suddenly fails, you have caught a regression (a change that broke something that used to work) before your users do.

We use two tools together:

• Jest — a test runner: it finds your test files, runs them, and reports pass/fail. (Vitest is a near-identical modern alternative; the syntax below works in both.)
• Supertest — sends fake HTTP requests straight into your Express app (no real network, no port), so you can check the response.

npm install --save-dev jest supertest

One small change makes testing possible

To test the app without actually opening a port, split building the app from starting it. Put your routes on app and export it; only call app.listen(...) in a separate entry file. Then a test can import the app directly.

// app.js — build the app and EXPORT it (no listen here)
const express = require("express");
const app = express();
app.use(express.json());

app.get("/tasks", (req, res) => res.json([{ id: 1, text: "Learn testing" }]));

module.exports = app;

Writing your first test

A test file ends in .test.js. Inside, test(name, fn) describes one case, and expect(value).toBe(...) checks a result. Supertest’s request(app).get("/tasks") sends a fake GET request and lets you inspect the response:

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

test("GET /tasks returns a 200 and a list", async () => {
  const res = await request(app).get("/tasks");

  expect(res.statusCode).toBe(200);
  expect(Array.isArray(res.body)).toBe(true);
  expect(res.body[0].text).toBe("Learn testing");
});

Reading it: request(app).get("/tasks") sends the request into your app and awaits the response. Then three checks (called assertions): the status code is 200, the body is an array, and the first item’s text is what we expect. If reality matches all three, the test passes; if any differs, Jest fails that line and shows what it got instead.

npx jest
# or add  "test": "jest"  to package.json scripts and run: npm test

Testing a POST and a failure case

Good test suites check both the happy path and the failures. Supertest can send a JSON body, and you should assert that bad input is rejected:

test("POST /tasks rejects an empty body with 400", async () => {
  const res = await request(app)
    .post("/tasks")
    .send({});                    // no text — should fail validation

  expect(res.statusCode).toBe(400);
});