What levels of transaction isolation do you know and what do they mean?

Main levels of transaction isolation:

• Read Uncommitted — the lowest isolation level, a transaction can see uncommitted changes of other transactions (dirty read).
• Read Committed — a transaction only sees data that has been committed by other transactions, preventing dirty reads, but non-repeatable reads are possible.
• Repeatable Read — guarantees that repeated reads of the same data within a transaction will be identical, preventing non-repeatable reads, but phantom reads are possible.
• Serializable — the highest isolation level, transactions are executed as if they were sequential, fully preventing phantom reads.

Example of usage in Node.js with PostgreSQL database via the pg library:

const { Client } = require('pg');

async function transactionExample() {
  const client = new Client();
  await client.connect();

  try {
    await client.query('BEGIN ISOLATION LEVEL SERIALIZABLE');
    // database operations
    await client.query('COMMIT');
  } catch (e) {
    await client.query('ROLLBACK');
    throw e;
  } finally {
    await client.end();
  }
}