为什么我的添加事件监听器为空？

Question

我的addEventListener在我的google开发工具上返回为null，但我不知道为什么。

var sx = document.getElementById('sx');        // Element to hold screenX
var sy = document.getElementById('sy');        // Element to hold screenY
var px = document.getElementById('px');        // Element to hold pageX
var py = document.getElementById('py');        // Element to hold pageY
var cx = document.getElementById('cx');        // Element to hold clientX
var cy = document.getElementById('cy');        // Element to hold clientY

function showPosition(event) {                 // Declare function
sx.value = event.screenX;                    // Update element with screenX
sy.value = event.screenY;                    // Update element with screenY
px.value = event.pageX;                      // Update element with pageX
py.value = event.pageY;                      // Update element with pageY
cx.value = event.clientX;                    // Update element with clientX
cy.value = event.clientY;                    // Update element with clientY
}

var el = document.getElementById('body');      // Get body Element
el.addEventListener('mousemove', showPosition, false); // Move updates  position

Answer 1

是否有一个元素的id="body"？

获取身体元素的更快方法

var x = document.body;