Testing a Node API

A manual test proves a route worked once. An automated test proves it still works after every change.

The Built-in Runner

Node includes a test runner. No extra package is required for simple tests. Save the file and run node --test.

import { test } from "node:test";
import assert from "node:assert/strict";
import { add } from "./maths.js";

test("adds two numbers", () => {
  assert.equal(add(2, 3), 5);
});

Test the Logic First

Extract validation and calculation into pure functions. Pure functions are fast to test and do not need a running server.

Test a Route

Start the app on a random port, send a request, assert the status and body, then close the server. Keep each test independent.

import { test } from "node:test";
import assert from "node:assert/strict";
import { app } from "./app.js";

test("GET /health returns ok", async () => {
  const server = app.listen(0);
  const { port } = server.address();

  const response = await fetch(`http://localhost:${port}/health`);
  const body = await response.json();

  assert.equal(response.status, 200);
  assert.equal(body.status, "ok");

  server.close();
});

Practice

1. Write a test for a function that formats a name.
2. Test that a missing student id returns 404.
3. Add npm test to package.json.

Hands-on exercise

Write tests for a function grade(score) that returns A, B, C or F.
1. Test a score of 72 returns A.
2. Test 64 returns B.
3. Test 41 returns F.
Run them with node --test.

Recap

Automated tests protect behaviour. Test pure logic first, then cover routes and the failure cases.