Two requests buy the last item — how do you avoid a race?

I don't do a separate SELECT, a check in JS and an unconditional UPDATE. I check availability and decrement the stock in one atomic statement.

-- inventory(product_id PRIMARY KEY, stock integer NOT NULL)
UPDATE inventory
SET stock = stock - 1
WHERE product_id = 42 AND stock > 0
RETURNING stock;

Result: no row returned means no such product or no stock left. Under the default Read Committed, a concurrent UPDATE waits and re-evaluates the condition after the competing change.

To be precise: if the order and the stock must be saved together, you need a transaction. An alternative for complex logic: SELECT FOR UPDATE and the update in the same transaction. Higher isolation may require retries.