Angular reusable functions

You should use a service, services has access to $rootScope, although is it better to keep DOM modification at directive level, in certain cases you can go for it.

angular.module("myModule", [])
.factory("MyService", function ($rootScope) {
    return {
        myFunction: function () { // do stuff here }
    }
})
.directive("MyDirective", function (MyService) {
    return {
        link: function (scope, iElement, iAttrs) {
            // try do keep dom modification here, you have access to MyService,
            // let it do the algorithm and try to keep dom modification here.
            // PS: you can also inject $rootScope to directives.
        };
    };
});

If this function needs to access Controller's scope, I would use the scope which is accessible in the directive and pass it as param, like so:

var MyDirective = function() {
    var linkFn = function(scope, element, attrs) { 
        callAwesomeFn(scope);
    };

    return {
        link: linkFn
    }
};

Now, where to put it?

• if this is utility function, I would put in some utilities namespace, like MyFunctions.callAwesomeFn()
• if this is a component which needs to interact with other objects managed by Angular - use Dependency Injection and inject it into directive