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如何在javascript中编写css代码？(未捕获的TypeError:无法设置undefined的属性"height")

JavaScript的

document.getElementById("slideshow").getElementsByClassName("arrow").style.height = "86px";

CSS

#slideshow .arrow{
    height:86px;
    width:60px;
    position:absolute;
    background:url('arrows.png') no-repeat;
    top:50%;
    margin-top: -43px;
    cursor: pointer;
    z-index: 5000;
}