angular $q, How to chain multiple promises within and after a for-loop

What you need to use is $q.all which combines a number of promises into one which is only resolved when all the promises are resolved.

In your case you could do something like:

function outerFunction() {

    var defer = $q.defer();
    var promises = [];

    function lastTask(){
        writeSome('finish').then( function(){
            defer.resolve();
        });
    }

    angular.forEach( $scope.testArray, function(value){
        promises.push(writeSome(value));
    });

    $q.all(promises).then(lastTask);

    return defer.promise;
}