diff --git a/parts/4/blogApp/package.json b/parts/4/blogApp/package.json
index 46a1109..66dab5e 100644
--- a/parts/4/blogApp/package.json
+++ b/parts/4/blogApp/package.json
@@ -5,7 +5,7 @@
   "scripts": {
     "start": "node src/app.js",
     "dev": "node --watch src/app.js",
-    "test": "echo \"Error: no test specified\" && exit 1"
+    "test": "node --test"
   },
   "author": "",
   "license": "ISC",
diff --git a/parts/4/blogApp/src/utils.js b/parts/4/blogApp/src/utils.js
new file mode 100644
index 0000000..30f4ac3
--- /dev/null
+++ b/parts/4/blogApp/src/utils.js
@@ -0,0 +1,16 @@
+const reverse = (string) => {
+  return string.split("").reverse().join("");
+};
+
+const average = (array) => {
+  const reducer = (sum, item) => {
+    return sum + item;
+  };
+
+  return array.length === 0 ? 0 : array.reduce(reducer, 0) / array.length;
+};
+
+module.exports = {
+  reverse,
+  average,
+};
diff --git a/parts/4/blogApp/tests/average.test.js b/parts/4/blogApp/tests/average.test.js
new file mode 100644
index 0000000..a7f56f5
--- /dev/null
+++ b/parts/4/blogApp/tests/average.test.js
@@ -0,0 +1,18 @@
+const { test, describe } = require('node:test')
+const assert = require('node:assert')
+
+const average = require('../src/utils').average
+
+describe('average', () => {
+  test('of one value is the value itself', () => {
+    assert.strictEqual(average([1]), 1)
+  })
+
+  test('of many is calculated right', () => {
+    assert.strictEqual(average([1, 2, 3, 4, 5, 6]), 3.5)
+  })
+
+  test('of empty array is zero', () => {
+    assert.strictEqual(average([]), 0)
+  })
+})
\ No newline at end of file
diff --git a/parts/4/blogApp/tests/reverse.test.js b/parts/4/blogApp/tests/reverse.test.js
new file mode 100644
index 0000000..59f044d
--- /dev/null
+++ b/parts/4/blogApp/tests/reverse.test.js
@@ -0,0 +1,22 @@
+const { test } = require('node:test')
+const assert = require('node:assert')
+
+const reverse = require('../src/utils.js').reverse
+
+test('reverse of a', () => {
+  const result = reverse('a')
+
+  assert.strictEqual(result, 'a')
+})
+
+test('reverse of react', () => {
+  const result = reverse('react')
+
+  assert.strictEqual(result, 'tcaer')
+})
+
+test('reverse of saippuakauppias', () => {
+  const result = reverse('saippuakauppias')
+
+  assert.strictEqual(result, 'saippuakauppias')
+})
\ No newline at end of file