Shape Guard.

Theory. Valid JSON is not the same as correct data. The text {"title":"Rent","amount":"500"} parses without error, but amount is a string and your code expects a number. [1,2] is valid JSON but not a record.

So after parse comes a second check — a "shape guard": the type of the value itself, required fields, and their types.

function isExpense(value) { if (typeof value !== "object" || value === null || Array.isArray(value)) return false; if (typeof value.title !== "string") return false; return typeof value.amount === "number" && Number.isFinite(value.amount); }

Only after the guard is it safe to use value.amount. Show a rejected value as "bad" and an accepted one as "ok".

Common mistake: typeof null is "object"; an array is also "object"; NaN is a number; accepting "500" as a number.