reentrancy.js

class Bank {
  constructor() {
    this.attacker_balance = 1;
  }

  withdraw() {
    if (this.attacker_balance == 1) {
      console.log('Sending ' + this.attacker_balance + ' ETH to attacker');
      send_money_to_attacker(this.attacker_balance);
    }
    console.log('Setting balance to 0');
    this.attacker_balance = 0;
  }
}

let bank = new Bank();
let withdrawals = 0;

function send_money_to_attacker(money) {
  console.log('Got ' + money + ' ETH');
  ++withdrawals;
  if (withdrawals < 10) {
    bank.withdraw();
  }
}

bank.withdraw();