Number.isNaN(): check NaN without hidden conversion

Useful to know

Number.isNaN() returns true only when the value submitted is the number NaN. For non-numeric values, returns false. This distinguishes it from the global function isNaN(), which can first convert the input. [1]

For example, Number.isNaN(Number('apple')) is true, because the conversion to a number failed. But Number.isNaN('apple') is false: the argument is text, not the number NaN. The two checks answer different questions.

When processing a field from a form, separate the steps: get the text, check if it is empty, convert it to a selected rule, and check the result. Do not assume that Number.isNaN() checks itself whether each input is an acceptable price or quantity.

There are other unacceptable numerical values according to the task: negative quantity, too large a number or infinity. They are not necessarily NaN. The business rule validation must indicate the permissible range separately.