<!DOCTYPE html><html><head><meta charset="utf-8"><title>Image Gallery</title><style type="text/css">*{margin: 0;padding: 0;}body{width: 960px;margin: 30px auto;background: rgba(134,205,80,0.2);}ul{display: inline-block;width: 100%;padding: 20px;}li{list-style: none;width: 20%;float: left;}a{text-decoration: none;color: #000;}img{width: 640px;height: 400px;}p{padding: 20px 0;}</style><script type="text/javascript">function addLoadEvent(func){var oldonload=window.onload;if(typeof window.onload!='function'){window.onload=func;}else{window.onload=function(){oldonload();func();}}}addLoadEvent(prepareGallery);function showPic(whichpic){if(!document.getElementById('placeholder')) return false;var source=whichpic.getAttribute('href');var placeholder=document.getElementById('placeholder');placeholder.setAttribute('src',source);if(document.getElementById('description')){var text=whichpic.getAttribute('title')?whichpic.getAttribute('title'):'';var description=document.getElementById('description');description.firstChild.nodeValue = text;}return true;}function prepareGallery(){if(!document.getElementsByTagName) return false;if(!document.getElementById) return false;if(!document.getElementById('gallery')) return false;var gallery=document.getElementById('gallery');var link=gallery.getElementsByTagName('a');for(var i=0;i<link.length;i++){link[i].onclick=function(){return !showPic(this);}}}</script></head><body><h1>Image Gallery</h1><ul id="gallery"><li><a href="img/2.jpg" title="image2" target="_blank">image2</a></li><li><a href="img/3.jpg" title="image3">image3</a></li><li><a href="img/4.jpg" title="image4">image4</a></li><li><a href="img/5.jpg" title="image5">image5</a></li></ul><img src="img/2.jpg" id="placeholder" alt="my image gallery"><p id="description">image gallery</p></body></html>关于addLoadEvent addLoadEvent(prepareGallery);这一操作处window.onload涉及到两个函数，一个是prepareGallery，一个是showPic，但为什么只有addLoadEvent(prepareGallery);没有addLoadEvent(showPic);呢？另外问一下，JS的执行顺序是怎样的呢？是不是先把所有的函数加载完毕。那这样的话，没有addLoadEvent(showPic);shoePic函数里的doucument.getElementById等操作应该获取不到，为什么会正常运行呢？如果在addLoadEvent(prepareGallery);后面加上addLoadEvent(showPic);代码依然可以正常运行，但是会显示index.html:60 Uncaught TypeError: Cannot read property 'getAttribute' of undefined at showPic (index.html:60) at window.onload (index.html:48)检查了一下也没有写错什么字母单词、标点符号，也没漏掉括号为什么会有错误麻烦大神解释一下这三个问题(图片麻烦修改后测试)
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