该atob函数将一个 Base64 编码的字符串解码为一个新的字符串，二进制数据的每个字节都有一个字符。

const byteCharacters = atob(b64Data);

每个字符的代码点 (charCode) 将是字节的值。我们可以通过使用.charCodeAt字符串中每个字符的方法应用它来创建一个字节值数组。

const byteNumbers = new Array(byteCharacters.length);
for (let i = 0; i < byteCharacters.length; i++) {
    byteNumbers[i] = byteCharacters.charCodeAt(i);
}

您可以将此字节值数组传递给Uint8Array构造函数，从而将其转换为真正的类型化字节数组。

const byteArray = new Uint8Array(byteNumbers);

这反过来可以通过将其包装在数组中并将其传递给Blob构造函数来转换为 BLOB 。

const blob = new Blob([byteArray], {type: contentType});

上面的代码有效。然而，性能可以通过byteCharacters在更小的切片中处理而不是一次处理来提高一点。在我的粗略测试中，512 字节似乎是一个不错的切片大小。这为我们提供了以下功能。

const b64toBlob = (b64Data, contentType='', sliceSize=512) => {
  const byteCharacters = atob(b64Data);
  const byteArrays = [];

  for (let offset = 0; offset < byteCharacters.length; offset += sliceSize) {
    const slice = byteCharacters.slice(offset, offset + sliceSize);

    const byteNumbers = new Array(slice.length);
    for (let i = 0; i < slice.length; i++) {
      byteNumbers[i] = slice.charCodeAt(i);
    }

    const byteArray = new Uint8Array(byteNumbers);
    byteArrays.push(byteArray);
  }

  const blob = new Blob(byteArrays, {type: contentType});
  return blob;
}

const blob = b64toBlob(b64Data, contentType);
const blobUrl = URL.createObjectURL(blob);

window.location = blobUrl;

完整示例：

const b64toBlob = (b64Data, contentType='', sliceSize=512) => {
  const byteCharacters = atob(b64Data);
  const byteArrays = [];

  for (let offset = 0; offset < byteCharacters.length; offset += sliceSize) {
    const slice = byteCharacters.slice(offset, offset + sliceSize);

    const byteNumbers = new Array(slice.length);
    for (let i = 0; i < slice.length; i++) {
      byteNumbers[i] = slice.charCodeAt(i);
    }

    const byteArray = new Uint8Array(byteNumbers);
    byteArrays.push(byteArray);
  }

  const blob = new Blob(byteArrays, {type: contentType});
  return blob;
}

const contentType = 'image/png';
const b64Data = 'iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg==';

const blob = b64toBlob(b64Data, contentType);
const blobUrl = URL.createObjectURL(blob);

const img = document.createElement('img');
img.src = blobUrl;
document.body.appendChild(img);

这是一个更简单的方法，没有任何依赖项或库。
它需要新的 fetch API。（可以用吗？）

var url = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg=="

fetch(url)
.then(res => res.blob())
.then(console.log)

使用此方法，您还可以轻松获取 ReadableStream、ArrayBuffer、文本和 JSON。
（仅供参考，这也适用于节点中的节点获取）

作为一个函数：

const b64toBlob = (base64, type = 'application/octet-stream') => 
  fetch(`data:${type};base64,${base64}`).then(res => res.blob())

我对 Jeremy 的 ES6 同步版本做了一个简单的性能测试。
同步版本将阻止 UI 一段时间。保持 devtool 打开会降低获取性能

document.body.innerHTML += '<input autofocus placeholder="try writing">'
// get some dummy gradient image
var img=function(){var a=document.createElement("canvas"),b=a.getContext("2d"),c=b.createLinearGradient(0,0,1500,1500);a.width=a.height=3000;c.addColorStop(0,"red");c.addColorStop(1,"blue");b.fillStyle=c;b.fillRect(0,0,a.width,a.height);return a.toDataURL()}();


async function perf() {
  
  const blob = await fetch(img).then(res => res.blob())
  // turn it to a dataURI
  const url = img
  const b64Data = url.split(',')[1]

  // Jeremy Banks solution
  const b64toBlob = (b64Data, contentType = '', sliceSize=512) => {
    const byteCharacters = atob(b64Data);
    const byteArrays = [];
    
    for (let offset = 0; offset < byteCharacters.length; offset += sliceSize) {
      const slice = byteCharacters.slice(offset, offset + sliceSize);
      
      const byteNumbers = new Array(slice.length);
      for (let i = 0; i < slice.length; i++) {
        byteNumbers[i] = slice.charCodeAt(i);
      }
      
      const byteArray = new Uint8Array(byteNumbers);
      
      byteArrays.push(byteArray);
    }
    
    const blob = new Blob(byteArrays, {type: contentType});
    return blob;
  }

  // bench blocking method
  let i = 500
  console.time('blocking b64')
  while (i--) {
    await b64toBlob(b64Data)
  }
  console.timeEnd('blocking b64')
  
  // bench non blocking
  i = 500

  // so that the function is not reconstructed each time
  const toBlob = res => res.blob()
  console.time('fetch')
  while (i--) {
    await fetch(url).then(toBlob)
  }
  console.timeEnd('fetch')
  console.log('done')
}

perf()

优化（但可读性较差）的实现：

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 });
}

对于所有浏览器支持，尤其是在 Android 上，也许您可​​以添加以下内容：

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
    }
}

看这个例子：https : //jsfiddle.net/pqhdce2L/

function b64toBlob(b64Data, contentType, sliceSize) {
  contentType = contentType || '';
  sliceSize = sliceSize || 512;

  var byteCharacters = atob(b64Data);
  var byteArrays = [];

  for (var offset = 0; offset < byteCharacters.length; offset += sliceSize) {
    var slice = byteCharacters.slice(offset, offset + sliceSize);

    var byteNumbers = new Array(slice.length);
    for (var i = 0; i < slice.length; i++) {
      byteNumbers[i] = slice.charCodeAt(i);
    }

    var byteArray = new Uint8Array(byteNumbers);

    byteArrays.push(byteArray);
  }
    
  var blob = new Blob(byteArrays, {type: contentType});
  return blob;
}


var contentType = 'image/png';
var b64Data = Your Base64 encode;

var blob = b64toBlob(b64Data, contentType);
var blobUrl = URL.createObjectURL(blob);

var img = document.createElement('img');
img.src = blobUrl;
document.body.appendChild(img);