js下载base64格式的图片

        <img name="pic1" />
        <input type="button" value="下载" onclick="download('img1')">
    <script>
        var ret="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";
          document.all['pic1'].src = "data:image/jpg;base64," + ret;  
//下载图片
        function download(){
                let imgData = "data:image/jpg;base64," + ret;  
                this.downloadFile('测试.png', imgData);
            }
      //下载
     function downloadFile(fileName, content) {
        let aLink = document.createElement('a');
        let blob = this.base64ToBlob(content); //new Blob([content]);

        let evt = document.createEvent("HTMLEvents");
        evt.initEvent("click", true, true);//initEvent 不加后两个参数在FF下会报错  事件类型，是否冒泡，是否阻止浏览器的默认行为
        aLink.download = fileName;
        aLink.href = URL.createObjectURL(blob);

        // aLink.dispatchEvent(evt);
        aLink.click()
      }
      //base64转blob
      function base64ToBlob(code) {
        let parts = code.split(';base64,');
        let contentType = parts[0].split(':')[1];
        let raw = window.atob(parts[1]);
        let rawLength = raw.length;

        let uInt8Array = new Uint8Array(rawLength);

        for (let i = 0; i < rawLength; ++i) {
          uInt8Array[i] = raw.charCodeAt(i);
        }
        return new Blob([uInt8Array], {type: contentType});
      }
    </script>