Having a quick read through the code (so I might have missed something) but the URL is generated from a substring of an SHA hash, does this not mean, at least in theory, URLs could be duplicated?
With a hex hash at 6 chars, there are a possible 16,777,216 unique URLs (Probably not an issue, unless you're bit.ly). But when there have been 1,677,721 URLs generated, there is a 10% chance a URL will be duplicated, at 8,388,608 there is a 50% chance, and so on. While those numbers are very high and probably not an issue, it's worth considering this probably wouldn't work on a large scale (Though I'm sure it wasn't designed too)
Not really a criticism, I still think this is a great example of how simple Node is, just an observation
With a hex hash at 6 chars, there are a possible 16,777,216 unique URLs (Probably not an issue, unless you're bit.ly). But when there have been 1,677,721 URLs generated, there is a 10% chance a URL will be duplicated, at 8,388,608 there is a 50% chance, and so on. While those numbers are very high and probably not an issue, it's worth considering this probably wouldn't work on a large scale (Though I'm sure it wasn't designed too)
Not really a criticism, I still think this is a great example of how simple Node is, just an observation