Note: This feature is available in Web Workers.
The size read-only property of the StylePropertyMapReadOnly interface returns a positive integer containing the size of the StylePropertyMapReadOnly object.
A positive integer.
Here we use the size property to return the total entries within the button elements computedStyleMap.
// grab our element
const buttonEl = document.querySelector("button");
// we can retrieve all computed styles with `computedStyleMap`
const allComputedStyles = buttonEl.computedStyleMap();
// use size to get the total styles within the map
const amountStyles = allComputedStyles.size;
console.log(amountStyles); // logs 338