Retrieve the value of a CSS rule for the specified element

Use Window.getComputedStyle() to get the value of the CSS rule for the specified element.

const getStyle = (el, ruleName) => getComputedStyle(el)[ruleName];

// EXAMPLE
getStyle(document.querySelector('p'), 'font-size'); // '16px'

Source

https://www.30secondsofcode.org/js/s/get-style