Azure Analytics UDF

I would like to know if it is possible to have an user defined .js, to decode Base64 from my Json String in Stream Analytics. And how free am I to use any js function or is there a preset number of functions which can be used. I tried using atob(), but SAnalytic tells me:

Function 'base64decod' resulted in an error: 'ReferenceError: 'atob' is not defined' Stack: ReferenceError: 'atob' is not defined at main (Unknown script code:3:4)

UDF:

function main(arg1) {
var decodData = atob(arg1);
return decodData;
}

Query:

SELECT
      [SystemProperties].[connectionDeviceId], temp, hum, rssi, snr, 
      EventProcessedUtcTime, UDF.base64decod(Body)
INTO
      [intoData]
FROM
      [fromBlob]

JSON Snippet:

{
 "EnqueuedTimeUtc": "2019-12-09T12:39:08.3320000Z",
 "Properties": {},
 "SystemProperties": {
   "to": "/devices/device-03/messages/events",
   "connectionDeviceId": "device-03",
   "connectionAuthMethod": "{\"scope\":\"device\",\"type\":\"sas\",\"issuer\":\"iothub\",\"acceptingIpFilterRule\":null}",
  "connectionDeviceGenerationId": "xxxxxxxxx025137526",
  "contentEncoding": "",
  "enqueuedTime": "2019-12-09T12:39:08.3320000Z"
},
 "Body": "eyJ0ZW1wIjoxLjc1LCJodW0iOjYxLjYzLCJyc3NpIjotMTAzLCJzbnIiOjguMn0=",
 "EventProcessedUtcTime": "2019-12-09T12:42:07.9089000Z",
 "PartitionId": 0,
 "BlobName": "measurements/iotc-xxxxxxxxxxxxxxxx/2019/12/09/12/40/01.json",
 "BlobLastModifiedUtcTime": "2019-12-09T12:41:06.0000000Z"
},

Thank you very much for your help!

According to official document,i'm afraid that you can't use any simple encapsulation method like atob or btoa. You could try some original things like below:

UDF code:

function base64decode(str) {
    var base64DecodeChars = new Array(
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,
    52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1,
    -1,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
    15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,
    -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
    41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1);

    var c1, c2, c3, c4;
    var i, len, out;

    len = str.length;
    i = 0;
    out = "";
    while(i < len) {
    /* c1 */
    do {
        c1 = base64DecodeChars[str.charCodeAt(i++) & 0xff];
    } while(i < len && c1 == -1);
    if(c1 == -1)
        break;

    /* c2 */
    do {
        c2 = base64DecodeChars[str.charCodeAt(i++) & 0xff];
    } while(i < len && c2 == -1);
    if(c2 == -1)
        break;

    out += String.fromCharCode((c1 << 2) | ((c2 & 0x30) >> 4));

    /* c3 */
    do {
        c3 = str.charCodeAt(i++) & 0xff;
        if(c3 == 61)
        return out;
        c3 = base64DecodeChars[c3];
    } while(i < len && c3 == -1);
    if(c3 == -1)
        break;

    out += String.fromCharCode(((c2 & 0XF) << 4) | ((c3 & 0x3C) >> 2));

    /* c4 */
    do {
        c4 = str.charCodeAt(i++) & 0xff;
        if(c4 == 61)
        return out;
        c4 = base64DecodeChars[c4];
    } while(i < len && c4 == -1);
    if(c4 == -1)
        break;
    out += String.fromCharCode(((c3 & 0x03) << 6) | c4);
    }
    return out;
}

SQL:

SELECT
      UDF.base64decode(Body)
FROM
      [fromBlob]

Output: