Below is a simple loan amortization calculator implemented in JavaScript. It's primarily intended to be used for home mortgages. To see the source code, or to make a comment, visit https://thiscouldbebetter.wordpress.com/2015/01/27/a-mortgage-calculator-in-javascript.

This doesn't qualify as professional financial advice, by the way. I tested some scenarios against a similar calculator, on bankrate.com, and while the numbers didn't match exactly, they were only ever off by a few cents. That's close enough for my purposes, but I'm told that creditors can be surprisingly petty about a couple of cents sometimes, so be careful.