The Daily WTF

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:

export function calcYears(date) { return date && Math.floor((new Date() – new Date(date).getTime()) / 3.15576e10) }

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

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