Change pseudo element value with a random element in Javascript

i want to change the value of a pseudo element with a random emoji. what i tried doesn't work. can you offer a solution?

css

.marquee span:after {
    content: attr(data-after);
}

javascript

const emojis = ['💵','💰','💳','💶','🐷','🐽','🐖','🔥','🖕', '😡', '🤬', '😠', '👿'];

const random = function () {
    randomEmoji = Math.floor(Math.random() * data.length)
}

const updateEmoji = function (ev) {
    const span = document.querySelector('.marquee span');
    ev.span.setAttribute('data-before', randomEmoji);
}