Yeah, I got a friend to write some example code for me that I tested out, but unfortunately is slightly broken. Soon I'm going to stop it from running on my VPS and make the git repo public. Will edit the OP.
What parts of the JavaScript do you need help with? By innerhtml, do you mean doing the live interaction with the user? Here's a super basic barebones example I put together with just BTC/USD that might help.
https://github.com/coinables/Bitcoin-USD-Price-Conversion-Tool/blob/master/js/price.js
I know the js to get the price to auto-update into a div, however for me it refuses it auto-update in the input box.
For now, I've set up a weighted average of multiple different exchanges and their bitcoin valurs, you can find it at /api/btc/usd.php and /api/usd/eur.php.
What I want the JS to do is to read the dropdown of the left text box and the value of the right dropdoen, and then get the output value from 52.62.67.167/api/*leftdropdown*/*rightdropdown*.php, multiply it by the left text box and spit it out into the right text box.
For example, to get 1 BTC = *** USD:
Fetch output from 52.62.67.167/api/btc/usd.php, multiply it by 1, then print it out where *** is.
Op,like the UI,what framework are you using? Pretty sleek!I can help you with the UI/Javascript,please update the code on github and inbox me once it's done.I've too much time on my hands lately.
Semantic UI for the dropdowns, everything else is my own.