大家好，我想要在網(wǎng)頁(yè)上作視差滾動(dòng)的效果，於是寫(xiě)了一段javascript插在html裡:<img class="graphic_001" src="img/parallax_graphic/tri-yellow.svg"/>                <script type="text/javascript">                         function parallex () {                        var img = window.pageYOffset;                        var a = document.getElementsByClassName('graphic_001');                        var speed = 0.8;                         a.style.top = img * speed +'px';                     }                    window.addEventListener('scroll',parallex);                </script>但是打開(kāi)瀏覽器之後，瀏覽器報(bào)錯(cuò)沒(méi)有抓到圖片的top，而且只要一滾動(dòng)就會(huì)報(bào)錯(cuò):想請(qǐng)問(wèn)應(yīng)該怎麼修正javascript呢?目前確認(rèn)圖片是有載到網(wǎng)頁(yè)上的，謝謝大家的回答!