问将append上的div 添加伪元素类？

var descDiv = document.createElement('div');

document.getElementById("overlay").appendChild(descDiv);

var cssStr = "background-color: rgba(255,255,255,.8);z-index:1088; position:absolute;left:" + Left + "px; top:" + Top + "px;";

descDiv.style.cssText = cssStr;

 .arrow_box {

    position: relative;

    border: 1px solid #c2e1f5;

    padding: 10px;

    width: 200px;

    height: 100px;

    border-radius: 6px;

    box-shadow: 0 1px 4px rgba(0, 0, 0, 0.3);

    margin: 50px;

    float: left;

}

.arrow_box:before,

.arrow_box:after {

    position: absolute;

    display: block;

    width: 0;

    height: 0;

    border: solid transparent;

    pointer-events: none;

    content: "";

    border-color: rgba(136, 183, 213, 0);

}

.arrow_box.left:before {

    border-left-color: #c2e1f5;

    border-width: 13px;

    top: 38%;

    left: 100%;

}

.arrow_box.left:after {

    border-left-color: #fff;

    border-width: 10px;

    top: 41%;

    left: 100%;

}