Get element's custom css properties (-mystyle) using JavaScript

Question

In an application where certain elements have custom CSS properties, is there any way to retrieve such a value via JavaScript?

e.g.

Answer 1

function getCustomCssProperty(elementID, propertyName){
  var style = document.getElementById(elementID).getAttribute("style");
  var entries = style.split(";");

 for (var i=0; i