Bridge: how can i shorten Metadata field to copy it

var Credit = decodeURI(selectedFile.name).substr(6, 8);

The other problem is that Iptc4xmpCore:SubjectCode is not a langAlt array, it's a bag array, so it was being written incorrectly.

I added a different substr that will work if your filenames are different lengths and are formatted [charactersYouDoNotwant][8 character IPTC SubjectCode].[3 character extension] 

#target bridge
if( BridgeTalk.appName == "bridge" ) {
FT = MenuElement.create("command", "Add FileName to IPTC Suject code", "at the end of Tools");
}
FT.onSelect = function () {
AddFilenameToCredit();
}
function AddFilenameToCredit(){
var thumbs = app.document.selections;
if(!thumbs.length) return;
if (ExternalObject.AdobeXMPScript == undefined) ExternalObject.AdobeXMPScript = new ExternalObject("lib:AdobeXMPScript");
for(var a in thumbs){
var selectedFile = thumbs[a].spec;
var str = decodeURI(selectedFile.name);
// works if filename is formatted [charactersYouDoNotwant][8 character IPTC SubjectCode].[3 character extension]  example: abcdef20000032.jpg
var Credit = str.substring(str.length-12, str.length-4);
// works if filename is always the same length
//var Credit = str.substr(6, 8);

var myXmpFile = new XMPFile( selectedFile.fsName, XMPConst.UNKNOWN, XMPConst.OPEN_FOR_UPDATE);
var myXmp = myXmpFile.getXMP();
myXmp.deleteProperty(XMPConst.NS_IPTC_CORE, "Iptc4xmpCore:SubjectCode");
myXmp.appendArrayItem(XMPConst.NS_IPTC_CORE, "Iptc4xmpCore:SubjectCode", Credit, 0, XMPConst.ARRAY_IS_UNORDERED);
if (myXmpFile.canPutXMP(myXmp)) {
myXmpFile.putXMP(myXmp);
myXmpFile.closeFile(XMPConst.CLOSE_UPDATE_SAFELY);
}
}
}