对于图像数据，我发现使用canvas更简单。toBlob(异步)

function b64toBlob(b64, onsuccess, onerror) {
    var img = new Image();

    img.onerror = onerror;

    img.onload = function onload() {
        var canvas = document.createElement('canvas');
        canvas.width = img.width;
        canvas.height = img.height;

        var ctx = canvas.getContext('2d');
        ctx.drawImage(img, 0, 0, canvas.width, canvas.height);

        canvas.toBlob(onsuccess);
    };

    img.src = b64;
}

var base64Data = 'data:image/jpg;base64,/9j/4AAQSkZJRgABAQA...';
b64toBlob(base64Data,
    function(blob) {
        var url = window.URL.createObjectURL(blob);
        // do something with url
    }, function(error) {
        // handle error
    });

优化(但可读性较差)实现:

function base64toBlob(base64Data, contentType) {
    contentType = contentType || '';
    var sliceSize = 1024;
    var byteCharacters = atob(base64Data);
    var bytesLength = byteCharacters.length;
    var slicesCount = Math.ceil(bytesLength / sliceSize);
    var byteArrays = new Array(slicesCount);

    for (var sliceIndex = 0; sliceIndex < slicesCount; ++sliceIndex) {
        var begin = sliceIndex * sliceSize;
        var end = Math.min(begin + sliceSize, bytesLength);

        var bytes = new Array(end - begin);
        for (var offset = begin, i = 0; offset < end; ++i, ++offset) {
            bytes[i] = byteCharacters[offset].charCodeAt(0);
        }
        byteArrays[sliceIndex] = new Uint8Array(bytes);
    }
    return new Blob(byteArrays, { type: contentType });
}

以下是我的TypeScript代码，可以很容易地转换成JavaScript，你可以使用

/**
 * Convert BASE64 to BLOB
 * @param base64Image Pass Base64 image data to convert into the BLOB
 */
private convertBase64ToBlob(base64Image: string) {
  // Split into two parts
  const parts = base64Image.split(';base64,');

  // Hold the content type
  const imageType = parts[0].split(':')[1];

  // Decode Base64 string
  const decodedData = window.atob(parts[1]);

  // Create UNIT8ARRAY of size same as row data length
  const uInt8Array = new Uint8Array(decodedData.length);

  // Insert all character code into uInt8Array
  for (let i = 0; i < decodedData.length; ++i) {
    uInt8Array[i] = decodedData.charCodeAt(i);
  }

  // Return BLOB image after conversion
  return new Blob([uInt8Array], { type: imageType });
}

对于所有像我一样喜欢复制粘贴的人来说，这里有一个可以在Chrome、Firefox和Edge上运行的现成下载功能:

window.saveFile = function (bytesBase64, mimeType, fileName) {
var fileUrl = "data:" + mimeType + ";base64," + bytesBase64;
fetch(fileUrl)
    .then(response => response.blob())
    .then(blob => {
        var link = window.document.createElement("a");
        link.href = window.URL.createObjectURL(blob, { type: mimeType });
        link.download = fileName;
        document.body.appendChild(link);
        link.click();
        document.body.removeChild(link);
    });
}

带fetch的方法是最好的解决方案，但如果有人需要使用一个没有fetch的方法，那么这里就是，因为前面提到的那些对我来说不适用:

function makeblob(dataURL) {
    const BASE64_MARKER = ';base64,';
    const parts = dataURL.split(BASE64_MARKER);
    const contentType = parts[0].split(':')[1];
    const raw = window.atob(parts[1]);
    const rawLength = raw.length;
    const uInt8Array = new Uint8Array(rawLength);

    for (let i = 0; i < rawLength; ++i) {
        uInt8Array[i] = raw.charCodeAt(i);
    }

    return new Blob([uInt8Array], { type: contentType });
}

对于所有浏览器的支持，特别是在Android上，也许你可以添加这个:

try{
    blob = new Blob(byteArrays, {type : contentType});
}
catch(e){
    // TypeError old Google Chrome and Firefox
    window.BlobBuilder = window.BlobBuilder ||
                         window.WebKitBlobBuilder ||
                         window.MozBlobBuilder ||
                         window.MSBlobBuilder;
    if(e.name == 'TypeError' && window.BlobBuilder){
        var bb = new BlobBuilder();
        bb.append(byteArrays);
        blob = bb.getBlob(contentType);
    }
    else if(e.name == "InvalidStateError"){
        // InvalidStateError (tested on FF13 WinXP)
        blob = new Blob(byteArrays, {type : contentType});
    }
    else{
        // We're screwed, blob constructor unsupported entirely
    }
}