diff --git a/Week4/assignment/Aggregation/.gitignore b/Week4/assignment/Aggregation/.gitignore
new file mode 100644
index 000000000..e78bbd2a3
--- /dev/null
+++ b/Week4/assignment/Aggregation/.gitignore
@@ -0,0 +1,20 @@
+# Node modules
+node_modules/
+
+# Logs
+*.log
+
+# Environment variables
+.env
+
+# Build output
+dist/
+build/
+
+# OS files
+.DS_Store
+Thumbs.db
+
+# IDE files
+.vscode/
+.idea/
diff --git a/Week4/assignment/Aggregation/ex1-aggregation.js b/Week4/assignment/Aggregation/ex1-aggregation.js
new file mode 100644
index 000000000..ff7546c6c
--- /dev/null
+++ b/Week4/assignment/Aggregation/ex1-aggregation.js
@@ -0,0 +1,94 @@
+import { MongoClient } from "mongodb";
+import dotenv from "dotenv" 
+
+dotenv.config()
+
+const url = process.env.MONGODB_URL
+if(!url) throw new Error("MONGODB_URL is not set in .env")
+const client = new MongoClient(url);
+const db = client.db("databaseWeek4");
+const collection = db.collection("week4Exercises");
+
+  /*Write a function that will return the array of the total population (M + F over all age groups) for a given Country per year. 
+    The result should look something like this, these are the values for Netherlands:*/
+
+async function getTotalPopulationByCountryPerYear() {
+  try {
+    // Example query: find first 1 document
+    const docs = await collection.find().limit(1).toArray();
+    console.log(docs);
+
+    const pipeline = [
+      { $match: { Country: "Netherlands" } },
+      {
+        $group: {
+          _id: "$Year",
+          countPopulation: { $sum: { $add: ["$M", "$F"] } },
+        },
+      },
+      { $sort: { _id: 1 } },
+    ];
+
+    const result = await collection.aggregate(pipeline).toArray();
+    console.log(result);
+
+    const formated = result.map((item) => {
+      return {
+        Year: item._id,
+        countPopulation: item.countPopulation,
+      };
+    });
+    console.table(formated);
+  } catch (err) {
+    console.error(err);
+  }
+}
+
+/*Write a function that will return all the information of each continent for a given Year 
+and Age field but add a new field TotalPopulation that will be the addition of M and F. 
+For example, if I would give 2020 for the Year and 100+ for the Age it should return something like this:*/
+async function getContinentPopulation(year, age) {
+  try {
+    const continents = [
+      "AFRICA",
+      "ASIA",
+      "EUROPE",
+      "LATIN AMERICA AND THE CARIBBEAN",
+      "NORTHERN AMERICA",
+      "OCEANIA",
+    ];
+    const pipeline = [
+      {
+        $match: { Year: year, Age: age, Country: { $in: continents } },
+      },
+      {
+        $addFields: {
+          TotalPopulation: { $add: ["$M", "$F"] },
+        },
+      },
+    ];
+
+    const result = await collection.aggregate(pipeline).toArray();
+    return result;
+  } catch (error) {
+    console.error("Error", error);
+  }
+}
+
+async function main() {
+  try {
+    await client.connect();
+    console.log("Connected to local MongoDB");
+    await getTotalPopulationByCountryPerYear();
+
+    const data = await getContinentPopulation(2020, "100+");
+    console.log(data);
+    console.table(data);
+  } catch (err) {
+    console.error(err);
+  } finally {
+    await client.close();
+  }
+}
+
+main();
diff --git a/Week4/assignment/Aggregation/package-lock.json b/Week4/assignment/Aggregation/package-lock.json
new file mode 100644
index 000000000..812697bd7
--- /dev/null
+++ b/Week4/assignment/Aggregation/package-lock.json
@@ -0,0 +1,176 @@
+{
+  "name": "aggregation",
+  "version": "1.0.0",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {
+    "": {
+      "name": "aggregation",
+      "version": "1.0.0",
+      "license": "ISC",
+      "dependencies": {
+        "dotenv": "^17.2.2",
+        "mongodb": "^6.19.0"
+      }
+    },
+    "node_modules/@mongodb-js/saslprep": {
+      "version": "1.3.0",
+      "resolved": "https://registry.npmjs.org/@mongodb-js/saslprep/-/saslprep-1.3.0.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": "11.0.5",
+      "resolved": "https://registry.npmjs.org/@types/whatwg-url/-/whatwg-url-11.0.5.tgz",
+      "integrity": "sha512-coYR071JRaHa+xoEvvYqvnIHaVqaYrLPbsufM9BF63HkwI5Lgmy2QR8Q5K/lYDYo5AK82wOvSOS0UsLTpTG7uQ==",
+      "license": "MIT",
+      "dependencies": {
+        "@types/webidl-conversions": "*"
+      }
+    },
+    "node_modules/bson": {
+      "version": "6.10.4",
+      "resolved": "https://registry.npmjs.org/bson/-/bson-6.10.4.tgz",
+      "integrity": "sha512-WIsKqkSC0ABoBJuT1LEX+2HEvNmNKKgnTAyd0fL8qzK4SH2i9NXg+t08YtdZp/V9IZ33cxe3iV4yM0qg8lMQng==",
+      "license": "Apache-2.0",
+      "engines": {
+        "node": ">=16.20.1"
+      }
+    },
+    "node_modules/dotenv": {
+      "version": "17.2.2",
+      "resolved": "https://registry.npmjs.org/dotenv/-/dotenv-17.2.2.tgz",
+      "integrity": "sha512-Sf2LSQP+bOlhKWWyhFsn0UsfdK/kCWRv1iuA2gXAwt3dyNabr6QSj00I2V10pidqz69soatm9ZwZvpQMTIOd5Q==",
+      "license": "BSD-2-Clause",
+      "engines": {
+        "node": ">=12"
+      },
+      "funding": {
+        "url": "https://dotenvx.com"
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+      "version": "1.5.0",
+      "resolved": "https://registry.npmjs.org/memory-pager/-/memory-pager-1.5.0.tgz",
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+      "license": "MIT"
+    },
+    "node_modules/mongodb": {
+      "version": "6.19.0",
+      "resolved": "https://registry.npmjs.org/mongodb/-/mongodb-6.19.0.tgz",
+      "integrity": "sha512-H3GtYujOJdeKIMLKBT9PwlDhGrQfplABNF1G904w6r5ZXKWyv77aB0X9B+rhmaAwjtllHzaEkvi9mkGVZxs2Bw==",
+      "license": "Apache-2.0",
+      "dependencies": {
+        "@mongodb-js/saslprep": "^1.1.9",
+        "bson": "^6.10.4",
+        "mongodb-connection-string-url": "^3.0.0"
+      },
+      "engines": {
+        "node": ">=16.20.1"
+      },
+      "peerDependencies": {
+        "@aws-sdk/credential-providers": "^3.188.0",
+        "@mongodb-js/zstd": "^1.1.0 || ^2.0.0",
+        "gcp-metadata": "^5.2.0",
+        "kerberos": "^2.0.1",
+        "mongodb-client-encryption": ">=6.0.0 <7",
+        "snappy": "^7.3.2",
+        "socks": "^2.7.1"
+      },
+      "peerDependenciesMeta": {
+        "@aws-sdk/credential-providers": {
+          "optional": true
+        },
+        "@mongodb-js/zstd": {
+          "optional": true
+        },
+        "gcp-metadata": {
+          "optional": true
+        },
+        "kerberos": {
+          "optional": true
+        },
+        "mongodb-client-encryption": {
+          "optional": true
+        },
+        "snappy": {
+          "optional": true
+        },
+        "socks": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/mongodb-connection-string-url": {
+      "version": "3.0.2",
+      "resolved": "https://registry.npmjs.org/mongodb-connection-string-url/-/mongodb-connection-string-url-3.0.2.tgz",
+      "integrity": "sha512-rMO7CGo/9BFwyZABcKAWL8UJwH/Kc2x0g72uhDWzG48URRax5TCIcJ7Rc3RZqffZzO/Gwff/jyKwCU9TN8gehA==",
+      "license": "Apache-2.0",
+      "dependencies": {
+        "@types/whatwg-url": "^11.0.2",
+        "whatwg-url": "^14.1.0 || ^13.0.0"
+      }
+    },
+    "node_modules/punycode": {
+      "version": "2.3.1",
+      "resolved": "https://registry.npmjs.org/punycode/-/punycode-2.3.1.tgz",
+      "integrity": "sha512-vYt7UD1U9Wg6138shLtLOvdAu+8DsC/ilFtEVHcH+wydcSpNE20AfSOduf6MkRFahL5FY7X1oU7nKVZFtfq8Fg==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=6"
+      }
+    },
+    "node_modules/sparse-bitfield": {
+      "version": "3.0.3",
+      "resolved": "https://registry.npmjs.org/sparse-bitfield/-/sparse-bitfield-3.0.3.tgz",
+      "integrity": "sha512-kvzhi7vqKTfkh0PZU+2D2PIllw2ymqJKujUcyPMd9Y75Nv4nPbGJZXNhxsgdQab2BmlDct1YnfQCguEvHr7VsQ==",
+      "license": "MIT",
+      "dependencies": {
+        "memory-pager": "^1.0.2"
+      }
+    },
+    "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"
+      },
+      "engines": {
+        "node": ">=18"
+      }
+    }
+  }
+}
diff --git a/Week4/assignment/Aggregation/package.json b/Week4/assignment/Aggregation/package.json
new file mode 100644
index 000000000..522dd063a
--- /dev/null
+++ b/Week4/assignment/Aggregation/package.json
@@ -0,0 +1,18 @@
+{
+  "name": "aggregation",
+  "version": "1.0.0",
+  "description": "",
+  "main": "ex1-aggregation.js",
+  "type": "module",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1",
+    "start": "node ex1-aggregation.js"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "dotenv": "^17.2.2",
+    "mongodb": "^6.19.0"
+  }
+}
diff --git a/Week4/assignment/Transaction/.gitignore b/Week4/assignment/Transaction/.gitignore
new file mode 100644
index 000000000..7fdb8c7cd
--- /dev/null
+++ b/Week4/assignment/Transaction/.gitignore
@@ -0,0 +1,20 @@
+# Node modules
+node_modules/
+
+# Logs
+logs/
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# Environment variables
+.env
+
+# OS files
+.DS_Store
+Thumbs.db
+
+# Optional: Build output
+dist/
+build/
diff --git a/Week4/assignment/Transaction/db.js b/Week4/assignment/Transaction/db.js
new file mode 100644
index 000000000..ee78bc9d3
--- /dev/null
+++ b/Week4/assignment/Transaction/db.js
@@ -0,0 +1,12 @@
+import { MongoClient } from "mongodb";
+import dotenv from "dotenv";
+
+dotenv.config();
+
+const MONGODB_URL = process.env.MONGODB_URL;
+if (!MONGODB_URL) throw new Error("MONGODB_URL is not defined in .env");
+const client = new MongoClient(MONGODB_URL);
+const db = client.db("Transfer");
+const transactionsCollection = db.collection("transactions");
+
+export { client, transactionsCollection };
diff --git a/Week4/assignment/Transaction/index.js b/Week4/assignment/Transaction/index.js
new file mode 100644
index 000000000..3d37e9b82
--- /dev/null
+++ b/Week4/assignment/Transaction/index.js
@@ -0,0 +1,20 @@
+import { client } from "./db.js";
+import { createAccounts, showAccounts } from "./setup.js";
+import { transferMoney } from "./transfer.js";
+
+async function connectToDatabase() {
+  try {
+    await client.connect();
+    console.log(`Successfully connected to MongoDB`);
+    await createAccounts();
+    await showAccounts();
+    await transferMoney(102, 103, 900);
+    await showAccounts();
+  } catch (error) {
+    console.error("Error:", error);
+  } finally {
+    await client.close();
+  }
+}
+
+connectToDatabase();
diff --git a/Week4/assignment/Transaction/package-lock.json b/Week4/assignment/Transaction/package-lock.json
new file mode 100644
index 000000000..75f23932f
--- /dev/null
+++ b/Week4/assignment/Transaction/package-lock.json
@@ -0,0 +1,176 @@
+{
+  "name": "transaction",
+  "version": "1.0.0",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {
+    "": {
+      "name": "transaction",
+      "version": "1.0.0",
+      "license": "ISC",
+      "dependencies": {
+        "dotenv": "^17.2.2",
+        "mongodb": "^6.19.0"
+      }
+    },
+    "node_modules/@mongodb-js/saslprep": {
+      "version": "1.3.0",
+      "resolved": "https://registry.npmjs.org/@mongodb-js/saslprep/-/saslprep-1.3.0.tgz",
+      "integrity": "sha512-zlayKCsIjYb7/IdfqxorK5+xUMyi4vOKcFy10wKJYc63NSdKI8mNME+uJqfatkPmOSMMUiojrL58IePKBm3gvQ==",
+      "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": "11.0.5",
+      "resolved": "https://registry.npmjs.org/@types/whatwg-url/-/whatwg-url-11.0.5.tgz",
+      "integrity": "sha512-coYR071JRaHa+xoEvvYqvnIHaVqaYrLPbsufM9BF63HkwI5Lgmy2QR8Q5K/lYDYo5AK82wOvSOS0UsLTpTG7uQ==",
+      "license": "MIT",
+      "dependencies": {
+        "@types/webidl-conversions": "*"
+      }
+    },
+    "node_modules/bson": {
+      "version": "6.10.4",
+      "resolved": "https://registry.npmjs.org/bson/-/bson-6.10.4.tgz",
+      "integrity": "sha512-WIsKqkSC0ABoBJuT1LEX+2HEvNmNKKgnTAyd0fL8qzK4SH2i9NXg+t08YtdZp/V9IZ33cxe3iV4yM0qg8lMQng==",
+      "license": "Apache-2.0",
+      "engines": {
+        "node": ">=16.20.1"
+      }
+    },
+    "node_modules/dotenv": {
+      "version": "17.2.2",
+      "resolved": "https://registry.npmjs.org/dotenv/-/dotenv-17.2.2.tgz",
+      "integrity": "sha512-Sf2LSQP+bOlhKWWyhFsn0UsfdK/kCWRv1iuA2gXAwt3dyNabr6QSj00I2V10pidqz69soatm9ZwZvpQMTIOd5Q==",
+      "license": "BSD-2-Clause",
+      "engines": {
+        "node": ">=12"
+      },
+      "funding": {
+        "url": "https://dotenvx.com"
+      }
+    },
+    "node_modules/memory-pager": {
+      "version": "1.5.0",
+      "resolved": "https://registry.npmjs.org/memory-pager/-/memory-pager-1.5.0.tgz",
+      "integrity": "sha512-ZS4Bp4r/Zoeq6+NLJpP+0Zzm0pR8whtGPf1XExKLJBAczGMnSi3It14OiNCStjQjM6NU1okjQGSxgEZN8eBYKg==",
+      "license": "MIT"
+    },
+    "node_modules/mongodb": {
+      "version": "6.19.0",
+      "resolved": "https://registry.npmjs.org/mongodb/-/mongodb-6.19.0.tgz",
+      "integrity": "sha512-H3GtYujOJdeKIMLKBT9PwlDhGrQfplABNF1G904w6r5ZXKWyv77aB0X9B+rhmaAwjtllHzaEkvi9mkGVZxs2Bw==",
+      "license": "Apache-2.0",
+      "dependencies": {
+        "@mongodb-js/saslprep": "^1.1.9",
+        "bson": "^6.10.4",
+        "mongodb-connection-string-url": "^3.0.0"
+      },
+      "engines": {
+        "node": ">=16.20.1"
+      },
+      "peerDependencies": {
+        "@aws-sdk/credential-providers": "^3.188.0",
+        "@mongodb-js/zstd": "^1.1.0 || ^2.0.0",
+        "gcp-metadata": "^5.2.0",
+        "kerberos": "^2.0.1",
+        "mongodb-client-encryption": ">=6.0.0 <7",
+        "snappy": "^7.3.2",
+        "socks": "^2.7.1"
+      },
+      "peerDependenciesMeta": {
+        "@aws-sdk/credential-providers": {
+          "optional": true
+        },
+        "@mongodb-js/zstd": {
+          "optional": true
+        },
+        "gcp-metadata": {
+          "optional": true
+        },
+        "kerberos": {
+          "optional": true
+        },
+        "mongodb-client-encryption": {
+          "optional": true
+        },
+        "snappy": {
+          "optional": true
+        },
+        "socks": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/mongodb-connection-string-url": {
+      "version": "3.0.2",
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+      "license": "Apache-2.0",
+      "dependencies": {
+        "@types/whatwg-url": "^11.0.2",
+        "whatwg-url": "^14.1.0 || ^13.0.0"
+      }
+    },
+    "node_modules/punycode": {
+      "version": "2.3.1",
+      "resolved": "https://registry.npmjs.org/punycode/-/punycode-2.3.1.tgz",
+      "integrity": "sha512-vYt7UD1U9Wg6138shLtLOvdAu+8DsC/ilFtEVHcH+wydcSpNE20AfSOduf6MkRFahL5FY7X1oU7nKVZFtfq8Fg==",
+      "license": "MIT",
+      "engines": {
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+      }
+    },
+    "node_modules/sparse-bitfield": {
+      "version": "3.0.3",
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+      "license": "MIT",
+      "dependencies": {
+        "memory-pager": "^1.0.2"
+      }
+    },
+    "node_modules/tr46": {
+      "version": "5.1.1",
+      "resolved": "https://registry.npmjs.org/tr46/-/tr46-5.1.1.tgz",
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+      "license": "MIT",
+      "dependencies": {
+        "punycode": "^2.3.1"
+      },
+      "engines": {
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+      }
+    },
+    "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"
+      },
+      "engines": {
+        "node": ">=18"
+      }
+    }
+  }
+}
diff --git a/Week4/assignment/Transaction/package.json b/Week4/assignment/Transaction/package.json
new file mode 100644
index 000000000..594583b14
--- /dev/null
+++ b/Week4/assignment/Transaction/package.json
@@ -0,0 +1,18 @@
+{
+  "name": "transaction",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "type": "module",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1",
+    "start": "node index.js"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "dotenv": "^17.2.2",
+    "mongodb": "^6.19.0"
+  }
+}
diff --git a/Week4/assignment/Transaction/setup.js b/Week4/assignment/Transaction/setup.js
new file mode 100644
index 000000000..53457d4bd
--- /dev/null
+++ b/Week4/assignment/Transaction/setup.js
@@ -0,0 +1,34 @@
+import { transactionsCollection } from "./db.js";
+export async function createAccounts() {
+  try {
+    await transactionsCollection.deleteMany({});
+    const accounts = [
+      {
+        account_number: 102,
+        balance: 5000,
+        account_changes: [],
+      },
+      {
+        account_number: 103,
+        balance: 10000,
+        account_changes: [],
+      },
+    ];
+
+    const result = await transactionsCollection.insertMany(accounts);
+
+    console.log(`Inserted ${result.insertedCount} accounts`);
+    return result;
+  } catch (error) {
+    console.error("Error creating accounts:", error);
+  }
+}
+
+export async function showAccounts() {
+  try {
+    const result = await transactionsCollection.find({}).toArray();
+    console.log(JSON.stringify(result, null, 2));
+  } catch (error) {
+    console.error("Error showing accounts:", error);
+  }
+}
diff --git a/Week4/assignment/Transaction/transfer.js b/Week4/assignment/Transaction/transfer.js
new file mode 100644
index 000000000..6f59b8ffc
--- /dev/null
+++ b/Week4/assignment/Transaction/transfer.js
@@ -0,0 +1,65 @@
+import { client, transactionsCollection } from "./db.js";
+
+export async function transferMoney(fromAccount, toAccount, amount) {
+  const sender = await transactionsCollection.findOne({
+    account_number: fromAccount,
+  });
+  const receiver = await transactionsCollection.findOne({
+    account_number: toAccount,
+  });
+
+  if (!sender || !receiver)
+    throw new Error("One of the accounts does not exist");
+  if (sender.balance < amount) throw new Error("Insufficient funds");
+
+  const senderChangeNumber =
+    (sender.account_changes.at(-1)?.change_number ?? 0) + 1;
+  const receiverChangeNumber =
+    (receiver.account_changes.at(-1)?.change_number ?? 0) + 1;
+
+  const remark = `Transferred ${amount} from account ${fromAccount} to account ${toAccount}`;
+
+  const session = client.startSession();
+  try {
+    await session.withTransaction(async () => {
+      await transactionsCollection.updateOne(
+        { account_number: fromAccount },
+        {
+          $inc: { balance: -amount },
+          $push: {
+            account_changes: {
+              change_number: senderChangeNumber,
+              amount: -amount,
+              changed_date: new Date(),
+              remark,
+            },
+          },
+        },
+        { session }
+      );
+
+      await transactionsCollection.updateOne(
+        { account_number: toAccount },
+        {
+          $inc: { balance: amount },
+          $push: {
+            account_changes: {
+              change_number: receiverChangeNumber,
+              amount: amount,
+              changed_date: new Date(),
+              remark,
+            },
+          },
+        },
+        { session }
+      );
+    });
+
+    console.log(remark);
+  } catch (error) {
+    console.error(" Transaction failed:", error.message);
+    await session.abortTransaction();
+  } finally {
+    await session.endSession();
+  }
+}