js将canvas保存成图片并下载

<canvas id="canvas" width="400" height="400"></canvas>
<div>
    <button id="save">保存</button>
</div>

var arr = [
    {locations:[[0,0],[200,200],[0,400]],color:"red"},
    {locations:[[0,0],[400,0],[200,200]],color:"orange"},
    {locations:[[0,400],[100,300],[200,400]],color:"yellow"},
    {locations:[[100,300],[200,200],[300,300],[200,400]],color:"green"},
    {locations:[[300,100],[200,200],[300,300]],color:"blue"},
    {locations:[[300,100],[400,0],[400,200],[300,300]],color:"indigo"},
    {locations:[[200,400],[400,400],[400,200]],color:"purple"}
];
var oCanvas = document.getElementById("canvas");
var ctx = oCanvas.getContext("2d");
for(let i=0;i<arr.length;i++){
    draw(arr[i],ctx);
}
 
function draw(item,ctx){
    ctx.beginPath();
    ctx.moveTo(item.locations[0][0],item.locations[0][1]);
    for(let i = 0;i<item.locations.length;i++){
        let x = item.locations[i][0];
        let y = item.locations[i][1];
        ctx.lineTo(x,y);
        console.log(1234567)
    }
    ctx.closePath();
     
    ctx.fillStyle = item.color;
    ctx.fill();
     
    ctx.strokeStyle = "#000";
    ctx.lineWidth = 2;
    ctx.stroke();
}
var imgId = document.getElementById("imgId");
document.getElementById("save").onclick = function (){
    downLoad(saveAsPNG(canvas));
}
 
// 保存成png格式的图片
function saveAsPNG(canvas) {
    return canvas.toDataURL("image/png");
}
 
// 保存成jpg格式的图片
function saveAsJPG(canvas) {
    return canvas.toDataURL("image/jpeg");
}
 
// 保存成bmp格式的图片  目前浏览器支持性不好
function saveAsBMP(canvas) {
    return canvas.toDataURL("image/bmp");
}
 
/**
 * @author web得胜
 * @param {String} url 需要下载的文件地址
 * */
function downLoad(url){
    var oA = document.createElement("a");
    oA.download = '';// 设置下载的文件名，默认是'下载'
    oA.href = url;
    document.body.appendChild(oA);
    oA.click();
    oA.remove(); // 下载之后把创建的元素删除
}