JS opacity animator

opacity.js

function sleep(ms) {
    return new Promise(resolve => setTimeout(resolve, ms));
}
async function opacity_to_100(id){
    var opacity = 0
    for (let i = 0; i < 25; i++) {
        document.getElementById(id).style.opacity = opacity+"%"
        opacity = opacity + 4
        await sleep(10)
    } 
}
async function opacity_to_0(id){
    var opacity = 100
    for (let i = 0; i < 25; i++) {
        document.getElementById(id).style.opacity = opacity+"%"
        opacity = opacity - 4
        await sleep(10)
    }
}