在浏览器中我们可以使用如下代码将base64转换层二进制数组。

function _base64ToArrayBuffer(base64) {
    var binary_string =  window.atob(base64);
    var len = binary_string.length;
    var bytes = new Uint8Array( len );
    for (var i = 0; i < len; i++)        {
        bytes[i] = binary_string.charCodeAt(i);
    }
    return bytes.buffer;
}

如果是小程序中等没有window对象的话就不行了。

虽然小程序提供wx.base64ToArrayBuffer(base64)，但是不知道为什么返回就是空。所以用如下代码来转换。

var Base64Binary = {
  _keyStr : "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=",
  
  /* will return a  Uint8Array type */
  decodeArrayBuffer: function(input) {
    var bytes = (input.length/4) * 3;
    var ab = new ArrayBuffer(bytes);
    this.decode(input, ab);
    
    return ab;
  },

  removePaddingChars: function(input){
    var lkey = this._keyStr.indexOf(input.charAt(input.length - 1));
    if(lkey == 64){
      return input.substring(0,input.length - 1);
    }
    return input;
  },

  decode: function (input, arrayBuffer) {
    //get last chars to see if are valid
    input = this.removePaddingChars(input);
    input = this.removePaddingChars(input);

    var bytes = parseInt((input.length / 4) * 3, 10);
    
    var uarray;
    var chr1, chr2, chr3;
    var enc1, enc2, enc3, enc4;
    var i = 0;
    var j = 0;
    
    if (arrayBuffer)
      uarray = new Uint8Array(arrayBuffer);
    else
      uarray = new Uint8Array(bytes);
    
    input = input.replace(/[^A-Za-z0-9\+\/\=]/g, "");
    
    for (i=0; i<bytes; i+=3) {  
      //get the 3 octects in 4 ascii chars
      enc1 = this._keyStr.indexOf(input.charAt(j++));
      enc2 = this._keyStr.indexOf(input.charAt(j++));
      enc3 = this._keyStr.indexOf(input.charAt(j++));
      enc4 = this._keyStr.indexOf(input.charAt(j++));
  
      chr1 = (enc1 << 2) | (enc2 >> 4);
      chr2 = ((enc2 & 15) << 4) | (enc3 >> 2);
      chr3 = ((enc3 & 3) << 6) | enc4;
  
      uarray[i] = chr1;      
      if (enc3 != 64) uarray[i+1] = chr2;
      if (enc4 != 64) uarray[i+2] = chr3;
    }
  
    return uarray;  
  }

var uintArray = Base64Binary.decode(base64_string);  
var byteArray = Base64Binary.decodeArrayBuffer(base64_string);