How to extract responseText from ajax error response?

In my go http handler function response to ajax request, I'd like to send an error message like this:

func AddBlogPOST(w http.ResponseWriter, r *http.Request) {
  //process request
  if everythingIsOk(r) {
      w.Write([]byte("ok"))
      return
   } else { 
    w.WriteHeader(http.StatusUnauthorized)
    w.Write([]byte("not enough credit"))
    return  
}

The ajax request is like this:

$(document).on('click', '#submit', function (e) { 
   $.ajax({
    url: '/blog/',
    type: 'post',
    data: $('.form').serialize(),
    success: (res) => {
      console.log('success body:', res);
        console.log(res);
       },
      error: (err) => {
        console.log('an error occured:', err);
      }
    });
  }
});

It works fine when the reuqest is successful and I get success body: ok in the browser console. However when there is an error, I get an object like:

readyState: 4, getResponseHeader: ƒ, getAllResponseHeaders: ƒ, setRequestHeader: ƒ, overrideMimeType: ƒ, …}
abort: ƒ (e)
always: ƒ ()
catch: ƒ (e)
done: ƒ ()
fail: ƒ ()
getAllResponseHeaders: ƒ ()
getResponseHeader: ƒ (e)
overrideMimeType: ƒ (e)
pipe: ƒ ()
progress: ƒ ()
promise: ƒ (e)
readyState: 4
responseText: "credit not enough"
setRequestHeader: ƒ (e,t)
state: ƒ ()
status: 401
statusCode: ƒ (e)
statusText: "Unauthorized"
then: ƒ (t,n,r)

How can I extract "credit not enough" from this object?

The argument named err in your error handler is the jqXHR object. To retrieve the error message you return from your server side code you need to access the responseText property of that object manually. Try this:

error: (jqXHR) => {
  console.log('an error occurred:', jqXHR.responseText);
}