js dom编程艺术这本书第四章4.4.1
代码：
function showPic (whichPic) {
var source = whichPic.getAttribute('href');
var placeholder = document.getElementById('placeholder');
placeholder.setAttribute('src',source);
}
function countBodyChildren(){
var body_element = document.getElementByTagName("body")[0];
alert(body_element.childNodes.length);
}
window.onload = countBodyChildren;
`请输入代码`应该有报错啊,拼错啦
是getElementsByTagName，少了个s