Form Field Format question

Hi All, 

If i apply a SSN(social Security number format) using below javascript, it allows alphabets to enter in the field, to correct that i have to manually set the format again in the field properties. I am not sure what makes the field to act like this. Below is the screenshot and JS .

var arr = ['clm_ssn', 'ee_ssn', 'patient_ssn', 'ssnp6',];
for (var i = 0; i < this.numFields; i++) {
var s = this.getField(this.getNthFieldName(i));
if (arr.indexOf(s.name) >= 0) {
s.setAction("Format", "AFSpecial_Format(3)");
s.setAction("Keystroke", "AFSpecial_KeystrokeEx(3)");
}
}