Primefaces Wizard skips the tab without validating fields

I created a javascript function to validate form fields made up of the <p:wizard> component of Primefaces. The function is called through the onnext="" event of the <p:wizard> , that is, only when the person clicks next (to go to the next tab). The problem is that my javascript function can not cause <p:wizard> not to skip the tab if the conditions are not met.

function onnext() {
var nome = document.getElementById('nome');
var cpf = document.getElementById('cpf');

if (nome.value === '') {
    alert('Por favor preencha o campo nome');
    nome.focus;
    return false;
} else if (cpf.value === '') {
    alert('Por favor preencha o campo cpf');
    cpf.focus;
    return false;
}
return true;}

I think the problem is in these return true and false ... but since I do not know what I should return, I ask for help here.

<p:wizard nextLabel="Próximo" onnext="onnext();" flowListener="#{alunoBean.onFlowProcess}">
<p:tab title="Dados Pessoais">
    <p:panel header="Dados Pessoais">
        <p:messages />                              
        <h:panelGrid columns="2">
            <p:outputLabel value="Nome Completo: " for="nome" />
            <p:inputText id="nome" value="#{alunoBean.aluno.nome}" />

            <p:outputLabel for="cpf" value="CPF: "/>
            <p:inputText  id="cpf" value="#{alunoBean.aluno.cpf}" />
        </h:panelGrid>
    </p:panel>
</p:tab>
<p:tab title="Dados Bancários">        
</p:tab>
<p:tab title="Dados Familiares">        
</p:tab>

With this javascript there, if you do not fill in the fields and click next, it shows alert(''); and jumps to the next tab. From here you can only go back, you can not jump to the next!