该demo放于tomcat下运行，否则出现跨域错误
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
</head>
<body style="height: 100%; margin: 0">
<div style="text-align: center;">
<canvas id="myCanvas" width="500" height="300" 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, 10, 10);
ctx.drawImage(img,50,50,100,100);
ctx.strokeStyle = "#0695FF"; //定义矩形的颜色
ctx.strokeRect(50,50,100,100);
ctx.strokeRect(50,170,100,100);
};
function imgType(ty) {
let type = ty.toLowerCase().replace(/jpg/i, ‘jpeg‘);
var r = type.match(/png|jpeg|bmp|gif/)[0];
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
原文：https://www.cnblogs.com/daysn/p/10374828.html