xts endpoints in Javascript

I couldn't easily find a pre-built method with javascript to do something like endpoints from the xts package in R. I am sure it exists, but I made this crude getEndPeriod function and thought I would apply here with S&P 500 data from the Federal Reserve Bank of St. Louis (FRED). Finding endpoints is important in finance to handle things like rebalancing.

d3.time.format('%Y') getQ getDecade

The formatting function supplied to getEndPeriod is flexible so it can use d3.js, moment.js, or any custom function that allows comparison to find breaks.