Asynchronously checking if email is taken already using AngularJS - angularjs

Question

I have the following email field for taking user input for email. Once the email has been entered and focus is taken away from the field ng-blur triggers the function that checks whether the email entered by the user has already been taken:
<input type="email" class="form-control" placeholder="" ng-model="email" name="email" required ng-blur="isFound(email)">
To show the error I've got the following span:
<span class="help-block error" ng-show="blurred && isTaken">Email is taken already</span>
And here is the function isFound(email):
$scope.isFound = function(email) {
service.isEmailTaken(email).then(function(data) {
console.log(data); //this shows either null or the returned data
$scope.blurred = true;
if(data == null) {
$scope.isTaken = false;
} else {
$scope.isTaken = true;
}
});
}
Now when I try an email that has been taken, it shows the message correctly. But after that even when I enter the email that has not been taken it keeps showing the same message that the Email is taken already.
Could somebody help me understand the reason why is it happening and how to resolve it?
EDIT - Adding AngularJS and NodeJS/ExpressJS/Mongoose code below:
AngularJS
factory.isEmailTaken = function(email) {
return $http({url: "/api/queries/email/" + email, method: "GET"}).then(function(resp) {
return resp.data;
});
};
ExpressJS/NodeJS/Mongoose
app.get('/api/queries/email/:email', function (req, res) {
Query.findOne({email: req.params.email}, function (err, query) {
if (err)
res.send(err);
res.json(query);
});
});

Answer 1

When the email was not taken already value of data was null, and the typeof data was String.
The reason why it was not working earlier was because if(data == null) was always turning out to be false.
if(data == 'null') got things working properly.