Prevent printing before filling out required fields

Hello,
for a form i added a print button, on it is a javascript. Only when I click on the button I get the error message that several required fields have to be filled in, but the printing process starts anyway.
What I have wrong with the code?

var emptyFields = [];

for (var i=0; i<this.numFields; i++) {

var f= this.getField(this.getNthFieldName(i));

if (f.type!="button" && f.required ) {

if ((f.type=="text" && f.value=="") || (f.type=="checkbox" && f.value=="Off")) emptyFields.push(f.name);

}

}

if (emptyFields.length>0) {

app.alert("Bitte füllen Sie noch folgende/s Pflichtfeld/er aus, bevor Sie das Formular drucken können:\n" + emptyFields.join("\n"));

}