Get all CSS variables on an HTML element with JavaScript when you don’t know the names of the CSS Variables?

If I know the name of a CSS variable eg --thing I can get it’s value with JavaScript:

  const elem = document.querySelector("html");
  if (!elem) return;
  const style = getComputedStyle(elem);
  const myVariable = style.getPropertyValue("--thing");
  console.log({ myVariable });

I’ve tried iterating over the properties and looking for names that start with -- however it doesn’t work.

  for (let i = 0; i < style.length; i++) {
    const propertyName = style[i];
    if (propertyName.startsWith("--")) {
      console.log(propertyName);
    }
  }