javascript - window.onload不执行？

Question

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`

Answer 1

是getElementsByTagName，少了个s

Answer 2

应该有报错啊,拼错啦

Answer 3

window.onload = countBodyChildren; 得到是9个孩子节点
window.onload = countBodyChildren();得到的是8个孩子节点，这是为什么？