Three things I would tell any NestJS developer before they write their first banking fraud detection rule

The first fraud rule anyone writes usually looks harmless. A quick check on the transaction amount, maybe an if statement, done. Then a second rule gets added for new recipients. Then a third for transaction frequency. Within a few weeks, one method has quietly grown into a long chain of conditions, each one dependent on the ones before it, and nobody feels confident touching it anymore.

I have seen this exact pattern happen more than once, and it is worth talking through before it happens to you, because the fix is simple if you catch it early, and genuinely painful to unwind once it has spread across a real production system.

First, stop adding rules as more conditions in one function

The instinct when a new fraud rule is needed is almost always to open the existing validation method and add one more if statement.

validateTransaction(dto: CreateTransactionDto) {