Local Variables

Local variable capture is the shortest path from stack trace to explanation. It tells you what values were in scope when the failure actually happened.

const rows = await db.query(
  "select * from invoices where status = ?",
  ["overdue"],
);

const invoice = rows[0];

if (!invoice) {
  throw new Error("disk quota exceeded");
}

In this case the useful signal is not just the thrown error. It is also that rows existed and invoice was undefined. That is often the difference between a fast diagnosis and a guess.