An example of how to Get Files from a Folder using Ecmascript\Javascript client object model in SharePoint

    <script type=”text/ecmascript”>
        function ViewAllFiles()
        {
            var context = new SP.ClientContext.get_current();
            var web = context.get_web();
            var list = web.get_lists().getByTitle(‘Shared Documents’);
            var query = SP.CamlQuery.createAllItemsQuery();
            query.set_folderServerRelativeUrl(‘/Shared%20Documents/TestFolder’);
            allItems = list.getItems(query);
            context.load(allItems, ‘Include(Title, ContentType, File)’);
            context.executeQueryAsync(Function.createDelegate(this, this.success), Function.createDelegate(this, this.failed));
        }

        function success()
        {
            var fileUrls = “”;
            var ListEnumerator = this.allItems.getEnumerator();
            while(ListEnumerator.moveNext())
            {
                var currentItem = ListEnumerator.get_current();
                var _contentType = currentItem.get_contentType();
                if(_contentType.get_name() != “Folder”)
                {
                    var File = currentItem.get_file();
                    if(File != null)
                    {
                        fileUrls += File.get_serverRelativeUrl() + ‘\n';
                    }
                }
            }
            alert(fileUrls);
        }

        function failed(sender, args) {
            alert("failed. Message:" + args.get_message());
        }
    </script>
    <a href="#" onclick="Javascript:ViewAllFiles();">View All Files in Test Folder</a>