Reconciling inbound cartons against a SKU list, in code

Inbound receiving is where fulfillment software is quietly bad. The core task is boring: compare what arrived against what you expected, and surface the diff. Model it as two lists and a join.

function reconcile(expected, received) {
  const bySku = new Map();
  for (const line of expected) bySku.set(line.sku, { expected: line.qty, received: 0 });
  for (const line of received) {
    const row = bySku.get(line.sku) || { expected: 0, received: 0 };
    row.received += line.qty;
    bySku.set(line.sku, row);
  }
  const diffs = [];
  for (const [sku, r] of bySku) {
    if (r.received !== r.expected) {
      diffs.push({ sku, expected: r.expected, received: r.received, delta: r.received - r.expected });
    }
  }
  return diffs; // empty means a clean receipt
}

A non-empty diffs array is the whole point: it is your exception queue.

• delta < 0 = short shipment.
• delta > 0 = over-ship or mislabeled carton.
• A SKU in received but not expected = mixed carton.

The discipline that makes this work is upstream: every inbound must be created from a SKU list, not free text. If receiving staff can type an arbitrary item, reconciliation is meaningless because the expected side is a guess.

Do this well and quality problems get caught at the dock, where they cost a phone call. Do it badly and they reach a customer, where they cost a refund and a bad review.