angularjs - Angular directive link function with 4 arguments? -

in angular developer guide find example:

var integer_regexp = /^\-?\d+$/; app.directive('integer', function() {   return {     require: 'ngmodel',     link: function(scope, elm, attrs, ctrl) {       ctrl.$parsers.unshift(function(viewvalue) {         if (integer_regexp.test(viewvalue)) {           // valid           ctrl.$setvalidity('integer', true);           return viewvalue;         } else {           // invalid, return undefined (no model update)           ctrl.$setvalidity('integer', false);           return undefined;         }       });     }   }; }); 

q) fourth argument in link function, , bind to? thought link function take 3 arguments.

it's controller or array of controllers, specified in required property of directive definition object. in particular case it's controller of ngmodel directive on same element

more info here