Javascript to get GUID from SharePoint

Here is an example using CSOM in JavaScript.

    var context = new SP.ClientContext.get_current();
    var listTitle = "Name of List";
    var viewTitle = "Name of View"; //You can compose a CAML or use a view.
        list = context.get_web().get_lists().getByTitle(listTitle); 
        view = list.get_views().getByTitle(viewTitle); 
        context.load(view); 

    context.executeQueryAsync( 
        function(sender, args) {getLinkListItem("<View><Query>" + view.get_viewQuery() + "</Query></View>")}, 
        function(sender, args) {alert("error: " + args.get_message());});}
    }, 'sp.js')});

 function getLinkListItem(camlQuery){
      var context = SP.ClientContext.get_current();
      var approvalList = context.get_web().get_lists().getByTitle(listTitle);

      var query = new SP.CamlQuery(); 
      query.set_viewXml(camlQuery); 

      listItems = approvalList.getItems(query);
      context.load(listItems);

      context.executeQueryAsync(ReadListItemSucceeded, ReadListItemFailed);
}
//Execute this when you receive the data back from SharePoint.
function ReadListItemSucceeded(sender, args) {
  var enumerator = listItems.getEnumerator();
  while (enumerator.moveNext()) {
    var listItem = enumerator.get_current();
    alert('The ID for ' +listItem.Title + ' is ' + listItem.id);
  }
}
//Execute this function if things go wrong.
function ReadListItemFailed(sender, args) {
  alert('Request failed.');
}

You should also take a look at the MSDN library entry for the JavaScript/ECMA CSOM.

Here is an example using a CAML query rather than a SharePoint view.

I'm not so familiar with java client object model for SharePoint, but I have worked with this ajax java script library (SPServices), this is quite useful, works with SharePoint web services and based on jQuery. It allows to query items and so on by calling functions like this:

    $().SPServices({
    operation: "GetListItems",
    async: false,
    listName: "Announcements",
    CAMLViewFields: "<ViewFields><FieldRef Name='Title' /></ViewFields>",
    completefunc: function (xData, Status) {
      $(xData.responseXML).find("[nodeName='z:row']").each(function() {
        var liHtml = "<li>" + $(this).attr("ows_Title") + "</li>";
        $("#tasksUL").append(liHtml);
      });
    }
  });

I am not absolutely clear what exactly you are trying to do. But if you upload files to document library (not as attachments to other list items), you could just retrieve the guid for the latest uploaded file for the current user. Is it what you are doing? Robert Kaucher gave some examples what you could do wit CSOM. Here is a working solution which I tested on my computer:

function getLatestDocumentGuid() {
    ///<summary>Gets the GUID for the latest uploaded document
    ///for current user
    ///</summary>
    var userId = _spPageContextInfo.userId;
    var caml = "<View><Query><Where>"
        + "<Eq><FieldRef Name='Author' LookupId='TRUE' /><Value Type='Integer'>" 
        + userId + "</Value></Eq></Where>" 
        + "<OrderBy><FieldRef Name='Created' Ascending='False' /></OrderBy>" 
        + "</Query><RowLimit>1</RowLimit></View>";
    var ctx = SP.ClientContext.get_current()
    var web = ctx.get_web()
    var list = web.get_lists().getByTitle("Documents")
    var query = new SP.CamlQuery();
    query.set_viewXml(caml);
    var items = list.getItems(query);
    ctx.load(items)
    ctx.executeQueryAsync(function() {
        // success actions
        var count = items.get_count();
        //should only be 1
        if (count > 1)  {
           throw "Something is wrong. Should only be one latest list item / doc";
        }

        var enumerator = items.getEnumerator();
        enumerator.moveNext();
        var item = enumerator.get_current();
        var guid = item.get_item("GUID");
        var guidString = guid.toString();
        // do something with your result!!!!
        alert(guidString);

    }, function() { 
        //failure handling comes here
        alert("failed"); 
    });
}
//run the function
getLatestDocumentGuid();

The CAML for the latest document/item comes from Rich Bennema's answer at SO. Using CAML in js CSOM comes from Rober Kaucher and MSDN. Getting the current user id in js comes from my blog