Javascript – jQuery Ajax – how to get response data in error

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I have a simple web application.
I've created the server REST API so it will return a response with HTTP code and a JSON (or XML) object with more details: application code (specific to scenario, message that describe what happened etc.).

So, for example if a client send a Register request and the password is too short, the response HTTP code will be 400 (Bad Request), and the response data will be: {appCode : 1020 , message : "Password is too short"}.

In jQuery I'm using the "ajax" function to create a POST request. When the server returns something different from HTTP code 200 (OK), jQuery defines it as "error".

The error handler can get 3 parameters: jqXHR, textStatus, errorThrown.
Ho can I get the JSON object that sent by the server in error case?

Edit:

1) Here is my JS code:

function register (userName, password) {
    var postData = {};
    postData["userName"] = userName;
    postData["password"] = password;

    $.ajax ({
        dataType: "json",
        type: "POST",
        url: "<server>/rest/register",
        data: postData,
        success: function(data) {
            showResultSucceed(data);
            hideWaitingDone();
        },
        error: function (jqXHR, textStatus, errorThrown) {

            showResultFailed(jqXHR.responseText);
            hideWaitingFail();
        }
    })
}

2) When looking at Firebug console, it seems like the response is empty.
When invoking the same request by using REST testing tool, I get a response with JSON object it it.

What am I doing wrong?

Best Answer

Here's an example of how you get JSON data on error:

$.ajax({
    url: '/path/to/script.php',
    data: {'my':'data'},
    type: 'POST'
}).fail(function($xhr) {
    var data = $xhr.responseJSON;
    console.log(data);
});

From the docs:

If json is specified, the response is parsed using jQuery.parseJSON before being passed, as an object, to the success handler. The parsed JSON object is made available through the responseJSON property of the jqXHR object.

Otherwise, if responseJSON is not available, you can try $.parseJSON($xhr.responseText).

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