点击a标签下载当前链接的图片&&js 通过 blob 类文件对象下载图片，修改图片保存的名字（兼容式写法）

convertUrlToBase64(url) {
return new Promise(function(resolve, reject) {
var img = new Image();
img.crossOrigin = "Anonymous";
img.src = url;
img.onload = function() {
var canvas = document.createElement("canvas");
canvas.width = img.width;
canvas.height = img.height;
var ctx = canvas.getContext("2d");
ctx.drawImage(img, 0, 0, img.width, img.height);
var ext = img.src
.substring(img.src.lastIndexOf(".") + 1)
.toLowerCase();
var dataURL = canvas.toDataURL("image/" + ext);
var base64 = {
dataURL: dataURL,
type: "image/" + ext,
ext: ext
};
resolve(base64);
};
});
},

convertBase64UrlToBlob(base64) {
var parts = base64.dataURL.split(";base64,");
var contentType = parts[0].split(":")[1];
var raw = window.atob(parts[1]);
var rawLength = raw.length;
var uInt8Array = new Uint8Array(rawLength);
for (var i = 0; i < rawLength; i++) {
uInt8Array[i] = raw.charCodeAt(i);
}
return new Blob([uInt8Array], { type: contentType });
},

myBrowser() {
var userAgent = navigator.userAgent; //取得浏览器的userAgent字符串
if (userAgent.indexOf("OPR") > -1) {
return "Opera";
} //判断是否Opera浏览器 OPR/43.0.2442.991
if (userAgent.indexOf("Firefox") > -1) {
return "FF";
} //判断是否Firefox浏览器  Firefox/51.0
if (userAgent.indexOf("Trident") > -1) {
return "IE";
} //判断是否IE浏览器 Trident/7.0; rv:11.0
if (userAgent.indexOf("Edge") > -1) {
return "Edge";
} //判断是否Edge浏览器 Edge/14.14393
if (userAgent.indexOf("Chrome") > -1) {
return "Chrome";
} // Chrome/56.0.2924.87
if (userAgent.indexOf("Safari") > -1) {
return "Safari";
} //判断是否Safari浏览器 AppleWebKit/534.57.2 Version/5.1.7 Safari/534.57.2
},

//图片格式和PDF
this.convertUrlToBase64(url).then(function(base64) {
// 图片转为base64
var blob = that.convertBase64UrlToBlob(base64); // 转为blob对象
// 下载
if (that.myBrowser() == "IE") {
window.navigator.msSaveBlob(blob, name + ".jpg");
} else if (that.myBrowser() == "FF") {
window.location.href = url;
} else {
var a = document.createElement("a");
a.download = name;
a.href = URL.createObjectURL(blob);
a.click();
}
});