diff --git a/Week4/homework/ex1-aggregation/index.js b/Week4/homework/ex1-aggregation/index.js
new file mode 100644
index 000000000..2a3583bde
--- /dev/null
+++ b/Week4/homework/ex1-aggregation/index.js
@@ -0,0 +1,102 @@
+const { MongoClient, ServerApiVersion } = require("mongodb");
+require("dotenv").config();
+
+const uri = process.env.MONGODB_URL;
+
+// Create a MongoClient with a MongoClientOptions object to set the Stable API version
+const client = new MongoClient(uri, {
+  serverApi: {
+    version: ServerApiVersion.v1,
+    strict: true,
+    deprecationErrors: true,
+  },
+});
+
+async function run() {
+  try {
+    // Connect the client to the server	(optional starting in v4.7)
+    await client.connect();
+    // Send a ping to confirm a successful connection
+    await client.db("databaseWeek4").command({ ping: 1 });
+    console.log(
+      "Pinged your deployment. You successfully connected to MongoDB!"
+    );
+
+    const mongoClient = client
+      .db("databaseWeek4")
+      .collection("population_pyramid_1950-2022");
+
+    console.log(await getPopulationPerCountry(mongoClient, "Netherlands"));
+
+    console.log(await getContinentInfo(mongoClient, 1990, "100+"));
+  } finally {
+    await client.close();
+  }
+}
+run().catch(console.dir);
+
+const getPopulationPerCountry = async (mongoClient, country) => {
+  const pipeline = [
+    {
+      $match: {
+        Country: country,
+      },
+    },
+    {
+      $group: {
+        _id: "$Year",
+        countPopulation: {
+          $sum: {
+            $add: ["$M", "$F"],
+          },
+        },
+      },
+    },
+    {
+      $project: {
+        _id: 0,
+        Year: "$_id",
+        countPopulation: 1,
+      },
+    },
+    {
+      $sort: {
+        Year: 1,
+      },
+    },
+  ]; //end pipeline
+
+  return await mongoClient.aggregate(pipeline).toArray();
+};
+
+const getContinentInfo = async (mongoClient, year, age) => {
+  const pipeline = [
+    {
+      $match: {
+        Country: {
+          $in: [
+            "AFRICA",
+            "ASIA",
+            "EUROPE",
+            "LATIN AMERICA AND THE CARIBBEAN",
+            "NORTHERN AMERICA",
+            "OCEANIA",
+          ],
+        },
+        Year: year,
+        Age: age,
+      },
+    },
+    {
+      $addFields: {
+        TotalPopulation: {
+          $sum: {
+            $add: ["$M", "$F"],
+          },
+        },
+      },
+    },
+  ]; //end pipeline
+
+  return await mongoClient.aggregate(pipeline).toArray();
+};
diff --git a/Week4/homework/ex1-aggregation/package-lock.json b/Week4/homework/ex1-aggregation/package-lock.json
new file mode 100644
index 000000000..6ca37528e
--- /dev/null
+++ b/Week4/homework/ex1-aggregation/package-lock.json
@@ -0,0 +1,179 @@
+{
+  "name": "ex1-aggregation",
+  "version": "1.0.0",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {
+    "": {
+      "name": "ex1-aggregation",
+      "version": "1.0.0",
+      "license": "ISC",
+      "dependencies": {
+        "dotenv": "^17.2.3",
+        "mongodb": "^7.0.0"
+      }
+    },
+    "node_modules/@mongodb-js/saslprep": {
+      "version": "1.4.0",
+      "resolved": "https://registry.npmjs.org/@mongodb-js/saslprep/-/saslprep-1.4.0.tgz",
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+      "license": "MIT",
+      "dependencies": {
+        "sparse-bitfield": "^3.0.3"
+      }
+    },
+    "node_modules/@types/webidl-conversions": {
+      "version": "7.0.3",
+      "resolved": "https://registry.npmjs.org/@types/webidl-conversions/-/webidl-conversions-7.0.3.tgz",
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+      "license": "MIT"
+    },
+    "node_modules/@types/whatwg-url": {
+      "version": "13.0.0",
+      "resolved": "https://registry.npmjs.org/@types/whatwg-url/-/whatwg-url-13.0.0.tgz",
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+      "license": "MIT",
+      "dependencies": {
+        "@types/webidl-conversions": "*"
+      }
+    },
+    "node_modules/bson": {
+      "version": "7.0.0",
+      "resolved": "https://registry.npmjs.org/bson/-/bson-7.0.0.tgz",
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+      "license": "Apache-2.0",
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+      "license": "Apache-2.0",
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+        "whatwg-url": "^14.1.0"
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+      "engines": {
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+      "license": "MIT",
+      "dependencies": {
+        "memory-pager": "^1.0.2"
+      }
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+    "node_modules/tr46": {
+      "version": "5.1.1",
+      "resolved": "https://registry.npmjs.org/tr46/-/tr46-5.1.1.tgz",
+      "integrity": "sha512-hdF5ZgjTqgAntKkklYw0R03MG2x/bSzTtkxmIRw/sTNV8YXsCJ1tfLAX23lhxhHJlEf3CRCOCGGWw3vI3GaSPw==",
+      "license": "MIT",
+      "dependencies": {
+        "punycode": "^2.3.1"
+      },
+      "engines": {
+        "node": ">=18"
+      }
+    },
+    "node_modules/webidl-conversions": {
+      "version": "7.0.0",
+      "resolved": "https://registry.npmjs.org/webidl-conversions/-/webidl-conversions-7.0.0.tgz",
+      "integrity": "sha512-VwddBukDzu71offAQR975unBIGqfKZpM+8ZX6ySk8nYhVoo5CYaZyzt3YBvYtRtO+aoGlqxPg/B87NGVZ/fu6g==",
+      "license": "BSD-2-Clause",
+      "engines": {
+        "node": ">=12"
+      }
+    },
+    "node_modules/whatwg-url": {
+      "version": "14.2.0",
+      "resolved": "https://registry.npmjs.org/whatwg-url/-/whatwg-url-14.2.0.tgz",
+      "integrity": "sha512-De72GdQZzNTUBBChsXueQUnPKDkg/5A5zp7pFDuQAj5UFoENpiACU0wlCvzpAGnTkj++ihpKwKyYewn/XNUbKw==",
+      "license": "MIT",
+      "dependencies": {
+        "tr46": "^5.1.0",
+        "webidl-conversions": "^7.0.0"
+      },
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+        "node": ">=18"
+      }
+    }
+  }
+}
diff --git a/Week4/homework/ex1-aggregation/package.json b/Week4/homework/ex1-aggregation/package.json
new file mode 100644
index 000000000..0b642de75
--- /dev/null
+++ b/Week4/homework/ex1-aggregation/package.json
@@ -0,0 +1,16 @@
+{
+  "name": "ex1-aggregation",
+  "version": "1.0.0",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "description": "",
+  "dependencies": {
+    "dotenv": "^17.2.3",
+    "mongodb": "^7.0.0"
+  }
+}
diff --git a/Week4/homework/ex2-transactions/index.js b/Week4/homework/ex2-transactions/index.js
new file mode 100644
index 000000000..8822a3179
--- /dev/null
+++ b/Week4/homework/ex2-transactions/index.js
@@ -0,0 +1,9 @@
+import { setup } from "./setup.js";
+import { transfer } from "./transfer.js";
+
+async function main() {
+  await setup();
+  await transfer(101, 102, 1000, "Transfer 1000 from 101 to 102");
+}
+
+main().catch(console.error);
diff --git a/Week4/homework/ex2-transactions/package-lock.json b/Week4/homework/ex2-transactions/package-lock.json
new file mode 100644
index 000000000..7b39673f9
--- /dev/null
+++ b/Week4/homework/ex2-transactions/package-lock.json
@@ -0,0 +1,179 @@
+{
+  "name": "ex2-transactions",
+  "version": "1.0.0",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {
+    "": {
+      "name": "ex2-transactions",
+      "version": "1.0.0",
+      "license": "ISC",
+      "dependencies": {
+        "dotenv": "^17.2.3",
+        "mongodb": "^7.0.0"
+      }
+    },
+    "node_modules/@mongodb-js/saslprep": {
+      "version": "1.4.0",
+      "resolved": "https://registry.npmjs.org/@mongodb-js/saslprep/-/saslprep-1.4.0.tgz",
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+      "license": "MIT",
+      "dependencies": {
+        "sparse-bitfield": "^3.0.3"
+      }
+    },
+    "node_modules/@types/webidl-conversions": {
+      "version": "7.0.3",
+      "resolved": "https://registry.npmjs.org/@types/webidl-conversions/-/webidl-conversions-7.0.3.tgz",
+      "integrity": "sha512-CiJJvcRtIgzadHCYXw7dqEnMNRjhGZlYK05Mj9OyktqV8uVT8fD2BFOB7S1uwBE3Kj2Z+4UyPmFw/Ixgw/LAlA==",
+      "license": "MIT"
+    },
+    "node_modules/@types/whatwg-url": {
+      "version": "13.0.0",
+      "resolved": "https://registry.npmjs.org/@types/whatwg-url/-/whatwg-url-13.0.0.tgz",
+      "integrity": "sha512-N8WXpbE6Wgri7KUSvrmQcqrMllKZ9uxkYWMt+mCSGwNc0Hsw9VQTW7ApqI4XNrx6/SaM2QQJCzMPDEXE058s+Q==",
+      "license": "MIT",
+      "dependencies": {
+        "@types/webidl-conversions": "*"
+      }
+    },
+    "node_modules/bson": {
+      "version": "7.0.0",
+      "resolved": "https://registry.npmjs.org/bson/-/bson-7.0.0.tgz",
+      "integrity": "sha512-Kwc6Wh4lQ5OmkqqKhYGKIuELXl+EPYSCObVE6bWsp1T/cGkOCBN0I8wF/T44BiuhHyNi1mmKVPXk60d41xZ7kw==",
+      "license": "Apache-2.0",
+      "engines": {
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+      "license": "BSD-2-Clause",
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+      },
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+      "version": "1.5.0",
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+    },
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+      "dependencies": {
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+        "whatwg-url": "^14.1.0"
+      },
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+      "integrity": "sha512-vYt7UD1U9Wg6138shLtLOvdAu+8DsC/ilFtEVHcH+wydcSpNE20AfSOduf6MkRFahL5FY7X1oU7nKVZFtfq8Fg==",
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+      "dependencies": {
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+      "integrity": "sha512-VwddBukDzu71offAQR975unBIGqfKZpM+8ZX6ySk8nYhVoo5CYaZyzt3YBvYtRtO+aoGlqxPg/B87NGVZ/fu6g==",
+      "license": "BSD-2-Clause",
+      "engines": {
+        "node": ">=12"
+      }
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+      "version": "14.2.0",
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+      "license": "MIT",
+      "dependencies": {
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+      }
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+  }
+}
diff --git a/Week4/homework/ex2-transactions/package.json b/Week4/homework/ex2-transactions/package.json
new file mode 100644
index 000000000..52e40eac9
--- /dev/null
+++ b/Week4/homework/ex2-transactions/package.json
@@ -0,0 +1,17 @@
+{
+  "name": "ex2-transactions",
+  "version": "1.0.0",
+  "main": "index.js",
+  "type": "module",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "description": "",
+  "dependencies": {
+    "dotenv": "^17.2.3",
+    "mongodb": "^7.0.0"
+  }
+}
diff --git a/Week4/homework/ex2-transactions/setup.js b/Week4/homework/ex2-transactions/setup.js
new file mode 100644
index 000000000..fb55743bc
--- /dev/null
+++ b/Week4/homework/ex2-transactions/setup.js
@@ -0,0 +1,76 @@
+import { MongoClient } from "mongodb";
+import "dotenv/config";
+
+const uri = process.env.MONGODB_URL;
+
+export async function setup() {
+  const client = new MongoClient(uri);
+
+  try {
+    await client.connect();
+    console.log("Connected to MongoDB!");
+
+    const db = client.db("databaseWeek4");
+    await insertAccountsCollection(db);
+  } catch (error) {
+    console.error("Connection error:", error);
+  } finally {
+    await client.close();
+  }
+}
+
+async function insertAccountsCollection(db) {
+  const accounts = db.collection("accounts");
+
+  // clean up
+  await accounts.drop().catch(() => {});
+
+  await accounts.insertMany([
+    {
+      account_number: 101,
+      balance: 2000,
+      account_changes: [
+        {
+          change_number: 1,
+          amount: 200,
+          changed_date: new Date("2025-01-20T10:15:00Z"),
+          remark: "Deposit",
+        },
+        {
+          change_number: 2,
+          amount: -100,
+          changed_date: new Date("2025-01-23T11:45:00Z"),
+          remark: "Online Purchase",
+        },
+      ],
+    },
+    {
+      account_number: 102,
+      balance: 500,
+      account_changes: [
+        {
+          change_number: 3,
+          amount: -50,
+          changed_date: new Date("2025-01-21T14:30:00Z"),
+          remark: "ATM Withdrawal",
+        },
+      ],
+    },
+    {
+      account_number: 103,
+      balance: 300,
+      account_changes: [
+        {
+          change_number: 4,
+          amount: 1000,
+          changed_date: new Date("2025-01-22T09:00:00Z"),
+          remark: "Salary Deposit",
+        },
+      ],
+    },
+    { account_number: 104, balance: 50, account_changes: [] },
+    { account_number: 105, balance: 0, account_changes: [] },
+  ]);
+
+  console.log("Inserted into accounts collection");
+}
diff --git a/Week4/homework/ex2-transactions/transfer.js b/Week4/homework/ex2-transactions/transfer.js
new file mode 100644
index 000000000..9b8d0ddfe
--- /dev/null
+++ b/Week4/homework/ex2-transactions/transfer.js
@@ -0,0 +1,86 @@
+import { MongoClient } from "mongodb";
+import "dotenv/config";
+
+const uri = process.env.MONGODB_URL;
+
+const client = new MongoClient(uri);
+
+export async function transfer(from, to, amount, remark) {
+  const session = client.startSession();
+  const db = client.db("databaseWeek4");
+  const accounts = db.collection("accounts");
+
+  const transactionOptions = {
+    readPreference: "primary",
+    readConcern: { level: "local" },
+    writeConcern: { w: "majority" },
+  };
+
+  try {
+    await client.connect();
+
+    const results = await session.withTransaction(async () => {
+      // get last change_number
+      const latest = await accounts
+        .aggregate(
+          [
+            { $unwind: "$account_changes" },
+            {
+              $group: {
+                _id: null,
+                maxChange: { $max: "$account_changes.change_number" },
+              },
+            },
+          ],
+          { session }
+        )
+        .toArray();
+
+      let next = (latest[0]?.maxChange ?? 0) + 1;
+
+      const senderResult = await accounts.updateOne(
+        { account_number: from },
+        {
+          $inc: { balance: -amount },
+          $push: {
+            account_changes: {
+              change_number: next,
+              amount: -amount,
+              changed_date: new Date(),
+              remark,
+            },
+          },
+        },
+        { session }
+      );
+
+      next++;
+
+      const recipientResult = await accounts.updateOne(
+        { account_number: to },
+        {
+          $inc: { balance: amount },
+          $push: {
+            account_changes: {
+              change_number: next,
+              amount,
+              changed_date: new Date(),
+              remark,
+            },
+          },
+        },
+        { session }
+      );
+
+      return { senderResult, recipientResult };
+    }, transactionOptions);
+
+    console.log("Transaction complete:", results);
+  } catch (error) {
+    console.error("Transaction aborted:", error);
+    await session.abortTransaction();
+  } finally {
+    await session.endSession();
+    await client.close();
+  }
+}