Testing JavaScript

Tests turn important expectations into executable examples. The best test level is the smallest one that proves behavior with confidence.

A focused unit test

01import test from "node:test";
02import assert from "node:assert/strict";
03import { progressPercent } from "./progress.js";
04
05test("calculates rounded progress", () => {
06  assert.equal(progressPercent(3, 8), 38);
07});
08
09test("handles an empty course", () => {
10  assert.equal(progressPercent(0, 0), 0);
11});

Use unit tests for calculation and parsing, integration tests for modules working together, and browser tests for critical user journeys.

Test outcomes, not implementation

• Give every test one observable reason to fail.
• Cover normal, boundary, failure, and accessibility behavior.
• Keep fixtures small and readable.
• Avoid asserting private function calls or internal markup details.
• Reproduce a bug with a failing test before fixing it.
• Run deterministic tests in continuous integration.

Keep this

Test public behavior, choose the cheapest trustworthy level, and make failures explain the broken expectation.