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

CSS:

:root {
    --custom-property: #000000;
}

Javascript:

var custom_property = window.getComputedStyle(document.body).getPropertyValue('--custom-property').trim()