Why should you care?

Express handlers chain multiple async steps; errors must propagate to centralized middleware.

See it live — copy this example

Paste into your MeanVerse project (Angular + Express + MongoDB), then run with ng serve / node / mongosh as noted.

async function settleTransfer(transferId: string) {
  const session = await mongoose.startSession();
  session.startTransaction();
  try {
    const transfer = await Transfer.findById(transferId).session(session);
    await Account.updateOne({ _id: transfer.from }, { $inc: { balance: -transfer.amount } }).session(session);
    await Account.updateOne({ _id: transfer.to }, { $inc: { balance: transfer.amount } }).session(session);
    transfer.status = 'completed';
    await transfer.save({ session });
    await session.commitTransaction();
  } catch (e) {
    await session.abortTransaction();
    throw e;
  } finally {
    session.endSession();
  }
}

What happened?

• Session groups Mongo ops.
• await each step sequentially inside transaction.
• finally always ends session even on failure.

Practice next

1. Wrap multi-step Mongoose work in try/finally.
2. Use util.promisify for legacy callback APIs.
3. Prefer async iterators for large Mongo cursors.
4. Convert settleTransfer to Promise chain for comparison.
5. Add p-timeout around axios bank verification call.

Remember

async/await is standard in MeanVerse API code. Always end Mongo sessions in finally. Streams help large export files.

ACH settlement window

Transfer must debit, credit, and audit log atomically.

Outcome: Async transaction code rolls back on any step failure.