setup node testing framework

package.json

@@ -6,8 +6,8 @@
   "scripts": {
     "start": "node src/index.js",
     "dev": "node --watch --env-file=.env src/index.js",
-    "test": "echo \"Error: no test specified\" && exit 1",
     "lint": "eslint src",
+    "test": "node --test --watch",
     "format": "prettier . -w"
   },
   "devDependencies": {

src/utils/test-demo.js

@@ -0,0 +1,11 @@
+export function reverse(string) {
+  return string.split('').reverse().join('')
+}
+
+export function average(array) {
+  if (!array.length) {
+    return 0
+  }
+
+  return array.reduce((sum, item) => sum + item, 0) / array.length
+}

src/utils/test-demo.test.js

@@ -0,0 +1,37 @@
+import assert from 'node:assert'
+import { describe, test } from 'node:test'
+import { average, reverse } from './test-demo.js'
+
+describe('reverse', () => {
+  test('reverse of a', () => {
+    const actual = reverse('a')
+    const expected = 'a'
+    assert.strictEqual(actual, expected)
+  })
+
+  test('reverse of react', () => {
+    const actual = reverse('react')
+    const expected = 'tcaer'
+    assert.strictEqual(actual, expected)
+  })
+
+  test('reverse of saippuakauppias', () => {
+    const actual = reverse('saippuakauppias')
+    const expected = 'saippuakauppias'
+    assert.strictEqual(actual, expected)
+  })
+})
+
+describe('average', () => {
+  test('of one value is the value itself', () => {
+    assert.strictEqual(average([1]), 1)
+  })
+
+  test('of many items is calculated right', () => {
+    assert.strictEqual(average([1, 2, 3, 4, 5, 6]), 3.5)
+  })
+
+  test('of empty array is zero', () => {
+    assert.strictEqual(average([]), 0)
+  })
+})