Validacao Formularios Javascript

-script>
function a(){
nome=document.form.txtnome.value;
if (nome.length==0) {
alert("Digite nome");
return false;
} {
alert("Postado");
return true;
}
-/script>
-body>
-form onsubmit="return a()" id="form" name="form" method="post" action="grava.php">
-label>
-input nome="txtnome" type="text" id="txtnome"/>
-/label>
-label>
-input type="submit" nome="submit" value="email"/>
-/label>