html2canvas 生成一个 base64 的海报

function downloadImage(base64Url) {
    let imgUrl = base64Url;
    if (window.navigator.msSaveOrOpenBlob) {//兼容IE浏览器的写法
	    let imageStr = atob(imgUrl.split(",")[1]);
	    let n = imageStr.length;
	    let u8arr = new Uint8Array(n);
	    while (n--) {
	    u8arr[n] = imageStr.charCodeAt(n);
	    }
	    let blob = new Blob([u8arr]);
	    window.navigator.msSaveOrOpenBlob(blob, "chart-download" + "." + "png");
	} else {//非IE浏览器
		let a = document.createElement("a");
		a.href = imgUrl;
		a.setAttribute("download", "chart-download");
		a.click();
	}
}