Get and set CSS styles, supporting camel-case, kebab-case, and custom properties

styles.js

function getStyle(el, prop) {
    const style = getComputedStyle(el);
    return prop.includes('-') ? style.getPropertyValue(prop) : style[prop];
}

function setStyle(el, prop, value) {
    if (prop.includes('-')) {
        el.style.setProperty(prop, value);
    } else {
        el.style[prop] = value;
    }
}

// Usage:

getStyle(element, 'borderWidth');
getStyle(element, 'border-width');
getStyle(element, '--custom-prop');

setStyle(element, 'backgroundColor', 'red');
setStyle(element, 'background-color', 'red');
setStyle(element, '--custom-prop', 'foo');