Backend code

backend/config.js

@@ -0,0 +1,3 @@
+const JWT_TOKEN = ""
+
+module.export = JWT_TOKEN

backend/db.js

@@ -0,0 +1,54 @@
+// backend/db.js
+const mongoose = require("mongoose");
+
+mongoose.connect("mongodb://localhost:27017/paytm");
+
+// Create a Schema for Users
+const userSchema = new mongoose.Schema({
+  username: {
+    type: String,
+    required: true,
+    unique: true,
+    trim: true,
+    lowercase: true,
+    minLength: 3,
+    maxLength: 30,
+  },
+  password: {
+    type: String,
+    required: true,
+    minLength: 6,
+  },
+  firstName: {
+    type: String,
+    required: true,
+    trim: true,
+    maxLength: 50,
+  },
+  lastName: {
+    type: String,
+    required: true,
+    trim: true,
+    maxLength: 50,
+  },
+});
+
+const accountSchema = new mongoose.Schema({
+  userId: {
+    type: mongoose.Schema.Types.ObjectId, // Reference to User model
+    ref: "User",
+    required: true,
+  },
+  balance: {
+    type: Number,
+    required: true,
+  },
+});
+
+const Account = mongoose.model("Account", accountSchema);
+const User = mongoose.model("User", userSchema);
+
+module.exports = {
+  User,
+  Account,
+};

backend/index.js

@@ -1,3 +1,13 @@
+// backend/index.js
 const express = require("express");
+const cors = require("cors");
+const rootRouter = require("./routes/index");
 
+const app = express();
 
+app.use(cors());
+app.use(express.json());
+
+app.use("/api/v1", rootRouter);
+
+app.listen(3000);

backend/middleware.js

@@ -0,0 +1,26 @@
+const { JWT_SECRET } = require("./config");
+const jwt = require("jsonwebtoken");
+
+const authMiddleware = (req, res, next) => {
+  const authHeader = req.headers.authorization;
+
+  if (!authHeader || !authHeader.startsWith("Bearer ")) {
+    return res.status(403).json({});
+  }
+
+  const token = authHeader.split(" ")[1];
+
+  try {
+    const decoded = jwt.verify(token, JWT_SECRET);
+
+    req.userId = decoded.userId;
+
+    next();
+  } catch (err) {
+    return res.status(403).json({});
+  }
+};
+
+module.exports = {
+  authMiddleware,
+};

backend/package.json

@@ -10,7 +10,9 @@
   "author": "",
   "license": "ISC",
   "dependencies": {
+    "cors": "^2.8.5",
     "express": "^4.18.2",
+    "jsonwebtoken": "^9.0.2",
     "mongoose": "^8.1.0",
     "zod": "^3.22.4"
   }

backend/routes/account.js

@@ -0,0 +1,63 @@
+// backend/routes/account.js
+const express = require("express");
+const { authMiddleware } = require("../middleware");
+const { Account } = require("../db");
+const { default: mongoose } = require("mongoose");
+
+const router = express.Router();
+
+router.get("/balance", authMiddleware, async (req, res) => {
+  const account = await Account.findOne({
+    userId: req.userId,
+  });
+
+  res.json({
+    balance: account.balance,
+  });
+});
+
+router.post("/transfer", authMiddleware, async (req, res) => {
+  const session = await mongoose.startSession();
+
+  session.startTransaction();
+  const { amount, to } = req.body;
+
+  // Fetch the accounts within the transaction
+  const account = await Account.findOne({ userId: req.userId }).session(
+    session 
+  );
+
+  if (!account || account.balance < amount) {
+    await session.abortTransaction();
+    return res.status(400).json({
+      message: "Insufficient balance",
+    });
+  }
+
+  const toAccount = await Account.findOne({ userId: to }).session(session);
+
+  if (!toAccount) {
+    await session.abortTransaction();
+    return res.status(400).json({
+      message: "Invalid account",
+    });
+  }
+
+  // Perform the transfer
+  await Account.updateOne(
+    { userId: req.userId },
+    { $inc: { balance: -amount } }
+  ).session(session);
+  await Account.updateOne(
+    { userId: to },
+    { $inc: { balance: amount } }
+  ).session(session);
+
+  // Commit the transaction
+  await session.commitTransaction();
+  res.json({
+    message: "Transfer successful",
+  });
+});
+
+module.exports = router;

backend/routes/index.js

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+// backend/user/index.js
+const express = require("express");
+const userRouter = require("./user");
+const accountRouter = require("./account");
+
+const router = express.Router();
+
+router.use("/user", userRouter);
+router.use("/account", accountRouter);
+
+module.exports = router;