> You should never use null in JavaScript for the value of a yet-to-determine number. Always use NaN. Actually, it is more idiomatic to use undefined for this

> You should never use null in JavaScript for the value of a yet-to-determine number. Always use NaN. Actually, it is more idiomatic to use undefined for this purpose rather than NaN... var determineLater; // undefined var blah = 5; blah += determineLater; // blah is now NaN