<!DOCTYPE html>

<html>

   <head>

       <meta charset="utf-8">

   </head>

   <body style="height: 100%; margin: 0">

        <div style="text-align: center;">

            <canvas id="myCanvas" width="" height="" style="border: 1px solid #fff123"></canvas>

            <div>

                <button id="btn" onclick="download()">点我下载</button>

            </div>

        </div>

       <script type="text/javascript">

   let cans=document.getElementById("myCanvas");

    let ctx=cans.getContext("2d");

    let img =new Image();

    img.src="2.jpg";

    img.setAttribute("crossOrigin",'Anonymous');

    img.onload = function(){

    //以Canvas画布上的坐标(10,10)为起始点，绘制图像

    ctx.drawImage(img, , );

    ctx.drawImage(img,,,,);

    ctx.strokeStyle = "#0695FF";        //定义矩形的颜色

    ctx.strokeRect(,,,);

    ctx.strokeRect(,,,);

     };

    function imgType(ty) {

        let type = ty.toLowerCase().replace(/jpg/i, 'jpeg');

        var r = type.match(/png|jpeg|bmp|gif/)[];

        return 'image/' + r;

    }

    function download() {

        let type = 'png';   //设置下载图片的格式

        let img_png_src = cans.toDataURL("image/png");  //将canvas保存为图片

        let imgData = img_png_src.replace(imgType(type),'image/octet-stream');

        let filename = '图片' + '.' + type; //下载图片的文件名

        saveFile(imgData,filename);

    }

    let saveFile = function(data, fileName){

        let save_link = document.createElement('a');

        save_link.href = data;

        save_link.download = fileName;

        let event = document.createEvent('MouseEvents');

        event.initEvent("click", true, false);

        save_link.dispatchEvent(event);

    };

       </script>

   </body>

</html