Extremely experimental at this point, and as of now, almost completely ported from GoldenCheetah (minus a few of their classes).
Only prints everything to sdout so far, but it's coming along pretty nicely.
You can use it in your own app with fitparse = require('fitparse').fitFileReader or you can run it from the command line:
fitparsejs -f /path/to/file.fit
More to come, especially after I get a chance to wrap my head around ANT's FIT SDK.