Making a $http call synchronous in AngularJS

General information for the website: AngularJS code modification
Kind of development: Customization of existing website
Description of every page/module: I have an existing running angularjs page and it makes api calls for field validation. I need someone to make the $http call synchronous.

Here is the function call:

function validateClass(classAbr){
if (!validationArr[classAbr]){
$http({
method: 'POST',
url: '/api/?action=validateClass',
data: 'stateAbr=' +classAbr,
headers: { 'Content-Type': 'application/x-www-form-urlencoded' }
}).
success(function(data, status, headers, config){
validationArr[classAbr] = data;
})
}

return validationArr[classAbr];
}
Description of requirements/features: You can use promise or any other method to make it synchronous, as long as it works I am fine with it.
Extra notes: