CAML Query for sub folder documents javascript

camlQuery.set_viewXml('<View Scope="Recursive"><Query></Query></View>');  

camlQuery.set_folderServerRelativeUrl(folderPath); 

In addition to specifying SP.CamlQuery.folderServerRelativeUrl, there is another way how to filter items by folder in CAML:

//Constructs a query that returns all items under specified folder                    
function createAllItemsInFolderQuery(folderUrl) {
    var qry = new SP.CamlQuery;
    var viewXml = "<View Scope='RecursiveAll'> \
                <Query> \
                    <Where> \
                        <Eq> \
                            <FieldRef Name='FileDirRef' /> \
                            <Value Type='Text'>" + folderUrl + "</Value> \
                        </Eq> \
                    </Where> \
                </Query> \
            </View>";
    qry.set_viewXml(viewXml);        
    return qry;
};

Note: FileDirRef column contains information about the file directory

Example: how to return documents from Documents library under Orders folder

var context = new SP.ClientContext.get_current();
var web = context.get_web();

var list = web.get_lists().getByTitle('Documents');
var qry = createAllItemsInFolderQuery('Shared Documents/Orders'); //Get documents under Orders folder (format: ListUrl/FolderUrl )
var items = list.getItems(qry);
context.load(items);

context.executeQueryAsync(
    function() {
        console.log(items.get_count()); //print number of documents in folder Orders        
    },
    function (sender, args) {
        console.log("Error: " + args.get_message());
    }
);