SOURCE

function loading(){
    console.log('loading')
    
    let loadingEl = document.querySelector('.mask')
    loadingEl.style.display = 'flex'
    setTimeout(()=>{
        loadingEl.style.display = 'none'
    }, 3000)
}

<button onclick="loading()">loading</button>
<p>这里有些文字，在加载的时候是选择不了的。</p>
<div class="mask">
    <div class="ball-scale"></div>
</div>

* {
    margin: 0;
    padding: 0;
}

.mask {
    position: fixed;
    top: 0;
    left: 0;
    z-index: 10000;
    width: 100vw;
    height: 100vh;
    background-color: rgba(0,0,0,.6);

    display: none;
    justify-content: center;
    align-items: center;
}