Building Banking-Grade Node.js APIs

When an API starts handling money movement, correctness becomes more important than feature velocity.

Non-negotiables

1. Explicit input contracts with clear validation errors.
2. Deterministic business rules with audited edge cases.
3. Structured logs and traceable request IDs.
4. Idempotency for any critical write operation.
5. Safe fallback behavior for downstream service failures.

A simple transaction guard

export function ensurePositiveAmount(amount: number) {
  if (!Number.isFinite(amount) || amount <= 0) {
    throw new Error("Amount must be a positive number.");
  }
}

Small guardrails like this are not enough alone, but they force correctness into every layer.

Final thought

Treat every external integration as a failure-prone dependency and every critical path as an incident that just has not happened yet.