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Mistake - comparison with optional chaining

I broke CSSBattle’s leaderboard recently because of an interesting bug. We had a piece of code, whose simplified version looked something like this:

const isMore = a.score > b.score;

This got recently changed so that score was now inside a key called overall and also, the overall key was optional - could be there or not. Also, in the old scenario score was always guaranteed to be there. But in the new structure, because original key itself isn’t guaranteed, hence the score key too. So very conveniently I changed the above comparison to access score from overall key through an optional chaining operator so that I don't get an error if overall key doesn’t exist:

const isMore = a.overall?.score > b.overall?.score;

Everything seemed to be working with the change…until I noticed that the comparison was returning faulty results in some cases.

The Bug

Here’s what happened. Assume in the original code, we have a.score as 40 and b.score as 0. With those values, isMore would come out to be true - which is correct. But what if now, a.original.score is 40, but b.orginal doesn’t exist. We expect isMore to be still true because 40 is more than nothing, right? But that doesn’t happen - isMore is false in the new case. Let’s see why.

Putting values in our expression, we get:

const isMore = 40 > undefined; // evaluates to false

Note: right side of the > operator would evaluate to undefined because of the optional chaining operator on a key which doesn’t exists. And 40 , though we expect to be more than something which doesn’t exists, is actually not more than undefined !

That makes sense too (now at least 😛). I say the value “doesn’t exists” a lot of times above, but undefined doesn’t mean “doesn’t exist”, it means the value isn’t defined. In JS, the closest to “doesn’t exists” would be null. And as should be expected, 40 > null actually is true!

Comparison with undefined gives false and null gives true, shown in devtools


I patched the above expression to have 0 as default value when original key doesn’t exist:

const isMore = (a.overall?.score || 0) > (b.overall?.score || 0);

Now, we won’t have undefined ever so numeric comparisons work correctly. Fixed!

I’ll see you again with my next mistake! 👋🙂