1. screen对象包含有关客户端显示屏幕的信息
    2. screen.availWidth
    3. screen.availHeight
    // 获取屏幕的宽度
var screenWidth = window.screen.availWidth;
console.log(screenWidth);

// 获取屏幕的可视区
var viewWidth = document.body.clientWidth;
console.log(viewWidth);