let canvas=document.querySelector('#page2 canvas'); //获取canvas标签

            let ctx=canvas.getContext('2d') //获取绘图环境

            let width=window.innerWidth;  //获取窗口宽度

            let height=window.innerHeight; //获取窗口高度

            canvas.width=width; //绘图高度为窗口高度

            canvas.height=height;//绘图宽度为窗口宽度