SOURCE

const oDiv = document.getElementsByClassName('container')[0]
const oBtn = document.getElementsByTagName('button')[0];
oBtn.addEventListener('click', function() {
    oDiv.classList.add('bfc')
})

<div class="container">
    <img src="https://gitee.com/caldey/BlogImage/raw/master/img/avatar.jpg"></img>
    <p class="bfc">Hello World</p>
</div>
<button>触发BFC</button>

.container {
    background-color: cornsilk 
}

img {
    float: left;
    width: 200px;
}

.bfc {
    overflow: hidden;
    /* display: inline-block; */
    /* display: flex; */
    /* position: fixed; */
    /* position: absolute; */
}