SyntaxError: Unexpected end of input at parse

Question

The following JS code snippet:

    var data_JSON = {
        input: 'test',
        message: 'Sending...'
    };

    $.ajax({
        url: 'main_php.php',
        type: 'POST',
        data: data_JSON,
        dataType : 'json',
        contentType: 'application/json',        
        success: alert(data),       
        error: function (request, status, error) {      
            alert(error);
        }
    }); 

And the following associated PHP code (main_php.php):

if ($_POST){    
    $vals = array(
        'input'     => $input,
        'message'   => $message
    );
    header('Content-Type: application/json');
    echo json_encode($vals);        
}

Always result error (error runs in $.ajax), whatever I tried. In the browser's developer console, I could explore the complete length of the error message:

SyntaxError: Unexpected end of input at parse (native) at ajaxConvert ([...]/jquery-3.0.0.js:8544:19) at done ([...]/jquery-3.0.0.js:9011:15) at XMLHttpRequest. ([...]/jquery-3.0.0.js:9303:9)

Which is exactly the point where the JSON is failed to parse? What is the reason of this error and how can I solve it?

Note: JSON.stringify did not work, if that helps anything.

Answer 1

try with:

js:

var data_JSON = {
    input: 'test',
    message: 'Sending...'
 };

$.ajax({
  url: 'main_php.php',
  method: 'POST',
  data: data_JSON,
  dataType : 'json',
  success: function(data){
    alert(JSON.stringify(data)) ;
  } ,       
  error: function (request, status, error) {      
    alert(error);
  }
}); 

and php:

if ($_POST){    
    $vals = array(
      'input'     => $input,
      'message'   => $message
    );
    header('Content-Type: application/json');
    echo json_encode($vals);        
}