$.每个jquery循环打印一个“;未定义的“;对于getJSON请求后的每个元素，网格数据都会完美地打印出来

我正在执行一个JSON请求，在循环元素之后，它们的行被完美地打印出来。问题是，我一直看到我正在构建的网格顶部的每个元素都有一个未定义的打印，我该如何处理它？。。。。。。

未定义未定义

 $("#newsletters").click(function(){
        $.getJSON("/newsletter/getNewsletters", function(result){
            var grid="<br><table id='newsletter_grid'>";
            grid+="<thead><tr><th></th><th>Año</th><th>Mes</th><th>Quincena</th><th></th><th></th><th>Publicado</th></tr></thead>";
             grid+="<tr><td><button id='agregar_button' onclick='crearNuevoNewsLetter()'>Nuevo</button></td></tr>";
             $.each(result, function(i, elem){
                    //show view fieldset, son los campos que se muestran para visualizar los datos
                    grid+=mostrarElementosViewMode(elem);               
                    //crea los hidden fields asociados a los view fields creados arriba de este
                    grid+=crearRowHidden(elem);

                });

            grid+="</table>";
            $("#grilla").html(grid);

        });


    }); 
function mostrarElementosViewMode(elem){
    var elemento;
    elemento+="<tr id='fieldset_"+elem.id+"'>";
    elemento+="<td><input type='hidden' value='"+elem.id+"'></td>";
    elemento+="<td id='anio_view_"+elem.id+"'>"+elem.anio+"</td>";
    elemento+="<td id='mes_view_"+elem.id+"'>"+elem.mes+"</td>";
    elemento+="<td id='quincena_view_"+elem.id+"'>"+elem.quincena+"</td>";
    elemento+="<td><button id='editar_"+elem.id+"' value='Editar' onclick='muestraEditarFieldsNewsletter('""+elem.id+"'")'>Editar</td>";
    elemento+="<td><button id='eliminar_newsletter_"+elem.id+"' value='Eliminar'>Eliminar</td>";
    if(elem.publicado == 1){
        elemento+="<td><input type='checkbox' id='publicado_view_"+elem.id+"' name='publicado' checked='checked'></td>";
    } else {
        elemento+="<td><input type='checkbox' id='publicado_view_"+elem.id+"' name='publicado'></td>";
    }
     elemento+="</tr>";
    return elemento;
}   
function crearRowHidden(elem){
    var nuevoHiddenFieldSet="";
    nuevoHiddenFieldSet+="<tr id='fieldset_hidden_"+elem.id+"' style='display:none'>";
    nuevoHiddenFieldSet+="<td><input type='hidden' value='"+elem.id+"'></td>";
    nuevoHiddenFieldSet+="<td><input name='anio' value='"+elem.anio+"' id='anio_"+elem.id+"' /></td>";
    nuevoHiddenFieldSet+="<td><input name='mes' value='"+elem.mes+"' id='mes_"+elem.id+"' /></td>";
    nuevoHiddenFieldSet+="<td><input name='quincena' value='"+elem.quincena+"' id='quincena_"+elem.id+"' /></td>";
    nuevoHiddenFieldSet+="<td><button value='Guardar' name='guardar_newsletter' id='guardar_newsletter_"+elem.id+"' onclick='guardarNewsletter("+elem.id+",'"update'")'>Guardar</td>";
    nuevoHiddenFieldSet+="<td></td>";
    if(elem.publicado==1){
        nuevoHiddenFieldSet+="<td><input type='checkbox' id='publicado_"+elem.id+"' name='publicado' checked='checked'></td>";
    }else{
        nuevoHiddenFieldSet+="<td><input type='checkbox' id='publicado_"+elem.id+"' name='publicado'></td>";
    }
    nuevoHiddenFieldSet+="</tr>";
    return nuevoHiddenFieldSet;
}

web服务返回此JSON:

[{"id":"12","anio":"2016","mes":"1","quincena":"4","publicado":"1"},{"id":"11","anio":"0000","mes":"1"
,"quincena":"1","publicado":"1"},{"id":"10","anio":"2000","mes":"1","quincena":"1","publicado":"1"},
{"id":"9","anio":"2000","mes":"1","quincena":"5","publicado":"1"},{"id":"8","anio":"0000","mes":"1","quincena"
:"1","publicado":"1"},{"id":"6","anio":"0000","mes":"1","quincena":"3","publicado":"1"},{"id":"5","anio"
:"2016","mes":"1","quincena":"5","publicado":"1"},{"id":"4","anio":"2016","mes":"1","quincena":"4","publicado"
:"1"},{"id":"3","anio":"2016","mes":"1","quincena":"3","publicado":"1"},{"id":"2","anio":"2016","mes"
:"1","quincena":"2","publicado":"1"},{"id":"1","anio":"2016","mes":"2","quincena":"1","publicado":"1"
}]