lib/angular/js/ng/directive/input.js
function validate(ctrl, validatorName, validity, value){
  ctrl.$setValidity(validatorName, validity);
  return validity ? value : undefined;
}
function validate(ctrl, validatorName, validity, value){
  ctrl.$setValidity(validatorName, validity);
  return validity ? value : undefined;
}
A helper function to call $setValidity and return the value / undefined, a pattern that is repeated a lot in the input validation logic.