tests/capitalize.test.js

 import { test } from 'node:test';

 import assert from 'node:assert';

 import { capitalize } from '../src/capitalize.js';

 test('capitalize normal string', () => {

  assert.strictEqual(capitalize('hello'), 'Hello');

 });

 test('capitalize empty string returns empty string', () => {

  assert.strictEqual(capitalize(''), '');

 });

 test('capitalize non-string input throws TypeError', () => {

  assert.throws(

  () => capitalize(123),

  {

  name: 'TypeError',

  message: 'Input must be a string',

  }

  );

 });