diff --git a/lab2/main_test.js b/lab2/main_test.js
index 5034468e..fa53c9b9 100644
--- a/lab2/main_test.js
+++ b/lab2/main_test.js
@@ -1,6 +1,121 @@
+const fs = require('fs');
 const test = require('node:test');
 const assert = require('assert');
-const { Application, MailSystem } = require('./main');
 
 // TODO: write your tests here
-// Remember to use Stub, Mock, and Spy when necessary
\ No newline at end of file
+// Remember to use Stub, Mock, and Spy when necessary
+
+// mock fs.readFile to simulate the file content before import main.js
+test.mock.method(fs, 'readFile', (path, encoding, callback) => {
+    callback(null, test_file_data);
+});
+const { Application, MailSystem } = require('./main');
+const { type } = require('os');
+
+// test data
+const test_people = ['Alice', 'Bob', 'Charlie'];
+const test_file_data = test_people.join('\n');
+
+test('MailSystem.write', () => {
+    const mailSystem = new MailSystem();
+    const name = test_people[0];
+    const expected = 'Congrats, ' + name + '!';
+
+    const actual = mailSystem.write(name);
+    assert.strictEqual(actual, expected);
+});
+
+test('MailSystem.send', () => {
+    const mailSystem = new MailSystem();
+    const name = test_people[0];
+    const context = 'Congrats, ' + name + '!';
+    
+    // use stub to simulate Math.random() with return value = 1
+    test.mock.method(Math, 'random', () => 1);
+    let actual = mailSystem.send(name, context);
+    assert.strictEqual(actual, true);
+
+    // use stub to simulate Math.random() with return value = 0
+    test.mock.method(Math, 'random', () => 0);
+    actual = mailSystem.send(name, context);
+    assert.strictEqual(actual, false);
+});
+
+test('Application.getNames', async () => { 
+    const application = new Application();
+
+    const [people, selected] = await application.getNames();
+    assert.deepStrictEqual(people, test_people); 
+    assert.deepStrictEqual(selected, []);
+});
+
+test('Application.getRandomPerson', async () => {
+    const application = new Application();
+    const [people, selected] = await application.getNames();
+
+    // use stub to simulate Math.random() with return value = 0
+    test.mock.method(Math, 'random', () => 0);
+    const person = application.getRandomPerson();
+    assert.strictEqual(person, test_people[0]);
+});
+
+test('Application.selectNextPerson', async () => {
+    const application = new Application();
+    const [people, selected] = await application.getNames();
+
+    // use stub to simulate application.getRandomPerson() with return value = test_people[0]
+    test.mock.method(application, 'getRandomPerson', () => {
+        return test_people[0];
+    });
+    const person = application.selectNextPerson();
+    assert.strictEqual(person, test_people[0]);
+    test.mock.restoreAll(); 
+    
+    // for the case when getRandomPerson returns the person that has already been selected
+    let dup = true;
+    test.mock.method(application, 'getRandomPerson', () => {
+        // return the person already selected once and return another person next time
+        if (dup) {
+            dup = false;
+            return test_people[0];
+        } else {
+            return test_people[1];
+        }
+    });
+    const person_after_dup = application.selectNextPerson();
+    test.mock.restoreAll();
+
+    // use stub to simulate application.getRandomPerson() with return value = test_people[2]
+    test.mock.method(application, 'getRandomPerson', () => {
+        return test_people[2];
+    });
+    
+    // select all people (adding the last one)
+    const last_person = application.selectNextPerson();
+
+    // when all people are already selected
+    const person_null = application.selectNextPerson();
+    assert.strictEqual(person_null, null);
+    
+});
+
+test ('Application.notifySelected', async () => {
+    const application = new Application();
+    const [people, selected] = await application.getNames();
+
+    test.mock.method(application, 'getRandomPerson', () => {
+        return test_people[0];
+    });
+    const person = application.selectNextPerson();
+
+    // use spy to check if mailSystem.write/send is called
+    const writeSpy = test.mock.fn(() => 'Congrats, ' + person + '!');
+    test.mock.method(application.mailSystem, 'write', writeSpy);
+    const sendSpy = test.mock.fn(() => true);
+    test.mock.method(application.mailSystem, 'send', sendSpy);
+
+    const actual = application.notifySelected();
+    assert.strictEqual(writeSpy.mock.calls.length, 1);
+    assert.strictEqual(sendSpy.mock.calls.length, 1);
+    assert.strictEqual(actual, undefined);
+});