Code Review: Race Condition & State Mutability

hardCode ReviewJS / TS

Problem Statement

Review the following wallet balance transfer handler. Identify how concurrent operations produce inconsistent state, explain the race condition window, and refactor using atomic operations or database transactions.

Code under review

JAVASCRIPT• handler.js

Pairlet Snippet

// Account balance transfer service
async function transferFunds(senderId, receiverId, amount) {
  const sender = await db.findUser(senderId);
  
  if (sender.balance < amount) {
    throw new Error("Insufficient funds");
  }

  await new Promise(resolve => setTimeout(resolve, 50));

  const receiver = await db.findUser(receiverId);

  const newSenderBalance = sender.balance - amount;
  const newReceiverBalance = receiver.balance + amount;

  await db.updateUserBalance(senderId, newSenderBalance);
  await db.updateUserBalance(receiverId, newReceiverBalance);

  return { success: true };
}

Read only. Write down what you would flag below, then compare with the reference notes.

JavaScript & TypeScript versions included

Run this as a live interview

Practice solo

Write down what you would flag in this code, then compare it with the reference notes.

Write your findings first, then check yourself against the notes.

Code Review: Race Condition & State Mutability

Problem Statement

Code under review

Practice solo

Get new interview exercises