Yes, negative numbers are truthy and that is necessary for this to work.

Both compareStats and sortStats list comparisons in priority order. The or "||" operator falls through to the next comparison only when the previous result was 0 (falsey), meaning the two items were considered equal by the previous comparison. A non-zero value (negative or positive) indicates we already know the relative order of the two items and don't need to look at further comparisons.

The result can then be used by the array sort method to actually sort the items. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort