JavaScript Practice 21/21

Capstone: JSON numbers that arrive as strings

This report has been right for months. This morning it mailed a customer an invoice whose total reads $19.9925.508.99. Nothing threw. No error, no alert - the total simply walked into a PDF and out to a human.

One line item changed. MS-204 now comes from a supplier whose serializer writes decimals as JSON strings ("25.50") rather than numbers, which real payment APIs genuinely do - a string cannot lose precision the way a float can. The other two lines still arrive as numbers. JSON.parse hands all three back exactly as written, because parsing is not validating: JSON has both types, and nothing in the format promises you which one you will get.

Then + decides. In JavaScript + is two operators sharing one symbol: add these numbers, or join these strings. If either side is a string, joining wins. So the reduce runs 0 + 19.99 and gets 19.99, then 19.99 + "25.50" and gets "19.9925.50", then joins 8.99 onto the end of that. Every step is legal. Every step is silent.

Number("25.50") gives you 25.5 and fixes it - but only halfway. Number("n/a") gives NaN, and NaN spreads through arithmetic just as quietly: sum + NaN is NaN, from there to the end, without a word. Coercing alone trades one silent wrong answer for another. So you coerce and check, at the edge where the data comes in - which turns a bad amount into a loud error you can locate, instead of a total nobody thinks to question.

Your task: fix invoiceTotal(json) so it returns the real total as a number, whatever mix of numbers and numeric strings the lines arrive in. If a line's amount is not a number at all, throw instead of returning a nonsense total. The sample payload is already in the editor - its true total is 54.48.

You'll practice:

Coercing and validating untrusted JSON at the boundary instead of trusting its types
Using typeof to prove a total became a string, because + joins the moment either side is one

const payload = '{"invoice":"INV-2026-0043","lines":[{"sku":"KB-101","amount":19.99},{"sku":"MS-204","amount":"25.50"},{"sku":"HD-330","amount":8.99}]}';

// Two lines arrive as JSON numbers, one arrives as a JSON string.
// This returns a total and never throws. Run it, then read what it printed.
function invoiceTotal(json) {
  const data = JSON.parse(json);
  return data.lines.reduce((sum, line) => sum + line.amount, 0);
}

console.log(`Invoice total: $${invoiceTotal(payload)}`);