Writing tests with Node's test runner

📘What it really is

• Node.js has a built-in test runner, so there's nothing to install. To test functions, first export them from their file (modules are covered fully in Module 29). At the end of calc.js:

module.exports = { lineTotal, subtotal, addTax };

• Then create calc.test.js:

const test = require("node:test");
const assert = require("node:assert/strict");
const { lineTotal, subtotal } = require("./calc.js");

test("lineTotal multiplies price by quantity", () => {
  assert.equal(lineTotal(6000, 2), 12000);
});

test("lineTotal rejects a zero quantity", () => {
  assert.throws(() => lineTotal(6000, 0));
});

test("subtotal of an empty cart is 0", () => {
  assert.equal(subtotal([]), 0);
});

• Run all tests with node --test. It finds files ending in .test.js and reports passes and failures.
• Useful checks:
• assert.equal(actual, expected) for single values;
• assert.deepEqual(actual, expected) for arrays and objects;

• assert.throws(() => ...) to check that bad input throws.

• Each test: arrange the input, act (call the function), assert the result.

• Popular test tools like Vitest and Jest work much the same way, with extra features. You'll use one later.