I did something similar with a spreadsheet years ago. It's lkudgy, but it works. You have to tab away from the input box and refresh the page, iirc.

https://gabrielsroka.github.io/webpages/calc.htm#a1:=Rate=3....

  https://gabrielsroka.github.io/webpages/calc.htm#a1:=Rate=3.875;a2:=Years=30;a3:=NPer=Years*12;a4:=PV=644000;a5:=Pmt=Math.round(Math.pmt(Rate/12/100,NPer,PV)*100+1)/100;rows:5;cols:1

It's about 130 js loc