故意在$ http发布请求中调用错误

I'm using Angular to send a post request with the $http service. I am doing all of the data validation of the form in Angular prior to sending the post request. However, there is one validation I am doing in PHP of whether the user already exists in the database. How do I purposefully invoke an error (in the php file) so that the Angular error callback is triggered instead of the success callback? Should I purposefully throw an exception?

IF the intent is to throw an exception, does the exception message get passed into the data parameter for the Angular error callback function?

Based on the comments to my question, I just did the following to my code:

if (duplicateUsers($username) > 0) {
  return http_response_code(400); // successfully generated an error in 
                                  // the $http AngularJS servicces
} else {
  // other code
}

You can chain your promises. The first promise will check the success content and that's also where you can throw an exception. This will cause the subsequent promises to return failure.

Here's a jsbin example.

angular
  .module('app', [])
  .run(function($http) {  
    var from$http = $http
      .get('www.google.com') //makes a request to www.google.com
      .then(function(response) {
        console.log('data was successfully retrieved from google');
        throw "from success handler"; //if has error, then throw "duplicated user"
      });

    from$http.then(function() { // this then block is handling the previous exception
      console.log('this success block is never called');
    }, function() {
      console.log('inside error block even tho success was returned from www.google.com');
    });
  });