index.js

const express = require("express");
const app = express();
const cors = require("cors");
const port = 3042;

app.use(cors());
app.use(express.json());

const balances = {
  "030a516fbcb67b1fe1cbd06f89bf1972d46efe12583d6bb759d8ac62f017b37b8b": 100,
  "034633f85e62385ae67846e10a2f676054e1708b600fc71f7207a38fff22be4fd0": 50,
  "02e47ac1ff6eee8067bb631666a796a97a175db1beae3975f8929e5f4405fb5369": 75,
};

app.get("/balance/:address", (req, res) => {
  const { address } = req.params;
  const balance = balances[address] || 0;
  res.send({ balance });
});

app.post("/send", (req, res) => {
  const { sender, recipient, amount } = req.body;

  setInitialBalance(sender);
  setInitialBalance(recipient);

  if (balances[sender] < amount) {
    res.status(400).send({ message: "Not enough funds!" });
  } else {
    balances[sender] -= amount;
    balances[recipient] += amount;
    res.send({ balance: balances[sender] });
  }
});

app.listen(port, () => {
  console.log(`Listening on port ${port}!`);
});

function setInitialBalance(address) {
  if (!balances[address]) {
    balances[address] = 0;
  }
}