CodeSOD: A Double Date

Alice picked up a ticket about a broken date calculation in a React application, and dropped into the code to take a look. There, she found this:

<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">calcYears</span>(<span class="hljs-params">date</span>) {
  <span class="hljs-keyword">return</span> date && <span class="hljs-title class_">Math</span>.<span class="hljs-title function_">floor</span>((<span class="hljs-keyword">new</span> <span class="hljs-title class_">Date</span>() - <span class="hljs-keyword">new</span> <span class="hljs-title class_">Date</span>(date).<span class="hljs-title function_">getTime</span>()) / <span class="hljs-number">3.15576e10</span>)
}

She stared at it for awhile, trying to understand what the hell this was doing, and why it was dividing by three billion. Also, why there was a && in there. But after staring at it for a few minutes, the sick logic of the code makes sense. getTime returns a timestamp in milliseconds. 3.15576e10 is the number of milliseconds in a year. So the Math.floor() expression just gets the difference between two dates as a number of years. The && is just a coalescing operator- the last truthy value gets returned, so if for some reason we can’t calculate the number of years (because of bad input, perhaps?), we just return the original input date, because that’s a brillant way to handle errors.

As bizarre as this code is, this isn’t the code that was causing problems. It works just fine. So why did Alice get a ticket? She spent some more time puzzling over that, while reading through the code, only to discover that this calcYears function was used almost everywhere in the code- but in one spot, someone decided to write their own.

<span class="hljs-keyword">if</span> (birthday) {
      <span class="hljs-keyword">let</span> year = birthday?.<span class="hljs-title function_">split</span>(<span class="hljs-string">'-'</span>, <span class="hljs-number">1</span>)
      <span class="hljs-keyword">if</span> (year[<span class="hljs-number">0</span>] != <span class="hljs-string">''</span>) {
        <span class="hljs-keyword">let</span> years = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Date</span>().<span class="hljs-title function_">getFullYear</span>() - year[<span class="hljs-number">0</span>]
        <span class="hljs-keyword">return</span> years
      }
}

So, this function also works, and is maybe a bit more clear about what it’s doing than the calcYears. But note the use of split– this assumes a lot about the input format of the date, and that assumption isn’t always reliable. While calcYears still does unexpected things if you fail to give it good input, its accepted range of inputs is broader. Here, if we’re not in a date format which starts with “YYYY-“, this blows up.

After spending hours puzzling over this, Alice writes:

I HATE HOW NO ONE KNOWS HOW TO CODE

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