A hack to get the value of a CSS style property in the provided unit

css-unit-conversion.js

// For certain CSS properties (width, height, etc.) getComputedStyle() will return the 
// "used value" (https://developer.mozilla.org/en-US/docs/Web/CSS/used_value) in pixels,
// regardless of what units it was explicitly defined with. This function allows you to
// get the value in any unit provided

function getStyleInUnit(el, prop, unit) {
    let value = parseFloat(getComputedStyle(el)[prop]) || 0;
    if (unit !== 'px') {
        el.style[prop] = 1 + unit;
        value = (1 / parseFloat(getComputedStyle(el)[prop])) * value;
        el.style[prop] = value + unit;
    }
    return value;
}

// Usage:

const el = document.querySelector('div'); //=> <div style="width: 5em"></div>

getComputedStyle(el).width; //=> "80px"

getStyleInUnit(el, 'width', 'em'); //=> 5
getStyleInUnit(el, 'width', '%');  //=> 4.203159725323512
getStyleInUnit(el, 'width', 'pt'); //=> 60.01267503168758