CodeSOD: Don't Not Be Negative

One of my favorite illusions is the progress bar. Even the worst, most inaccurate progress bar will make an application feel faster. The simple feedback which promises "something is happening" alters the users' sense of time.

So, let's say you're implementing a JavaScript progress bar. You need to decide if you are "in progress" or not. So you need to check: if there is a progress value, and the progress value is less than 100, you're still in progress.

Mehdi's co-worker decided to implement that check as... the opposite.

const isInProgress = progress => !(!progress || (progress && progress > 100))

This is one of those lines of code where you can just see the developer's process, encoded in each choice made. "Okay, we're not in progress if progress doesn't have a value: !(!progress). Or we're not in progress if progress has a value and that value is over 100."

There's nothing explicitly wrong with this code. It's just the most awkward, backwards possible way to express that check. I suspect that part of its tortured logic arises from the fact that the developer wanted to return false if the value was null or undefined, and this was the way they figured out to do that.

Of course, a more straightforward way to write that might be (progress) => (progress && progress <= 100) || false. This will have "unexpected" behavior if the progress value is negative, but then again, so will the original code.

In the end, this is just a story of a double negative. I definitely won't say you should never not use a double negative. Don't not avoid them.

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