StylePropertyMapReadOnly.size

The size read-only property of the StylePropertyMapReadOnly interface returns an unsinged long integer containing the size of the StylePropertyMapReadOnly object.

var size = StylePropertyMapReadOnly.size

An unsigned long 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