// This section loads the contents of the base64 string. The active example
// loads the current job file.
var fso = new ActiveXObject("Scripting.FileSystemObject");
var source_file = fso.OpenTextFile(Watch.GetJobFilename(), 1);
var base64_contents = source_file.ReadAll();

// Then we call the function below. This setup writes back to
// the current job file, but it can be a path on the disk (second parameter)
// The third parameter determines the mime type, which can be image/jpg, image/png, etc.
// It has to correspond to what's present in the base64 string!
base64ToBinary(base64_contents, Watch.GetJobFilename(), "image/jpeg");

// This is the magic function that "cheats" by writing the
// base64 string directly into a binary node. MAGIC FTW.
// This is extremely fast and efficient, by the way.
function base64ToBinary(base64String, filePath, mimeType) {
  var dom = new ActiveXObject('Microsoft.XMLDOM');
  var elem = dom.createElement('tmp');
  elem.dataType = 'bin.base64';
  base64String = base64String.replace("data:"+mimeType+";base64,", "");
  elem.text = base64String;
  var decodeBase64 = elem.nodeTypedValue

  var inputStream = new ActiveXObject('ADODB.Stream');
  inputStream.Open();
  inputStream.Type = 1;  // adTypeBinary
  inputStream.Write(decodeBase64);
  inputStream.SaveToFile(filePath, 2);
}

Getting the string from another source:

Here are some other sources you can use to obtain the base64 data:

// Load a local variable's content as the base64 string:
// var base64_contents = Watch.GetVariable("encodedImage");
// 
// Load the data from the XML envelope of the HTTP Server Input:
// var base64_contents = Watch.ExpandString("xmlget('/request[1]/values[1]/photo-data[1]',Value,KeepCase,NoTrim)");

The same trick can be used in VBScript, using pretty much the same “xml binary node” cheat as above:

Dim objFSO, objFile, strBase64
Set objFSO = CreateObject("Scripting.FileSystemObject")
Set objFile = objFSO.OpenTextFile(Watch.GetJobFileName, 1)
strBase64 = objFile.ReadAll

objFile.Close

byte64ToJpeg strBase64, Watch.GetJobFileName

Private Sub byte64ToJpeg(byte64string,jpegpath)
        dim DM, EL,decodeBase64
        Set DM = CreateObject("Microsoft.XMLDOM")
        Set EL = DM.createElement("tmp")
        EL.DataType = "bin.base64"
        EL.Text = Replace(byte64string,"data:image/jpeg;base64,", "")
        decodeBase64 = EL.NodeTypedValue
        Dim binaryStream
        Set binaryStream = CreateObject("ADODB.Stream")
        binaryStream.Type = 1
        binaryStream.Open
        binaryStream.Write decodeBase64
        binaryStream.SaveToFile jpegpath, 2
        set EL = nothing
        set DM = nothing
        set binaryStream = Nothing
end sub