JavaScript POST请求问题

Question

So I'm creating a mobile application using Intel XDK, I am sending a JavaScript POST request of a username and password. I also did some research on the HTTP status codes and found this:

200 OK - Standard response for successful HTTP requests. The actual response will depend on the request method used. In a GET request, the response will contain an entity corresponding to the requested resource. In a POST request the response will contain an entity describing or containing the result of the action.

201 Created - The request has been fulfilled and resulted in a new resource being created.

202 Accepted - The request has been accepted for processing, but the processing has not been completed. The request might or might not eventually be acted upon, as it might be disallowed when processing actually takes place.

So I would assume that when a new user is inserted through a POST request the status would be 201. However when I had this code:

        XMLHTTP.onreadystatechange = function () {
            if (XMLHTTP.status == 201) {
                alert("User created.");
            } else {
                alert("Error!");
            }
        }

It would show the "Error!" and not "User created." But when I added this on to the code:

        XMLHTTP.onreadystatechange = function () {
            if (XMLHTTP.status == 201 || XMLHTTP.status == 200) {
                alert("User created.");
            } else {
                alert("Error!");
            }
        }

It showed "User created." So I was wondering how come the status is 200 even though I'm sending a POST request to insert in to a database.

Secondly, it alerts the "User created." 4 times? Is that because it is in the function onreadystatechange so it changes each time and is alerted? If so I can I make it so that it only alerts one? Should I have an if statement wrapped in a setinterval as shown below:

setInterval(function () {
    if (XMLHTTP.status == StatusNumberHere) {
        alert("Blah");
    }
}, 10);

Answer 1

Very few websites use those headers, your back-end probably just sends a 200 even though the request was successful in inserting data.

About your second question, the reason your alert is triggered four times is because onreadystatechanged is called four times, each with a different readyState :

1. Server connection established
2. request received
3. processing request
4. request finished and response is ready

So you probably want to add XMLHTTP.readyState === 4 to your if statement so it in the end becomes:

    XMLHTTP.onreadystatechange = function () {
        if (XMLHTTP.status === 200 && XMLHTTP.readyState === 4) {
            alert("User created.");
        } else {
            alert("Error!");
        }
    }

Answer 2

The status returned is based on how the server decides to handle it, in most cased you will simply get a success (200), so there is no issue or abnormality with what you have done.

setInterval = hack, avoid at all costs unless implementing some actual interval function. You can check the the readystate of the XMLHTTP request with

XMLHTTP.readyState == 4

to filter out only the completed event.

Here is a list of the ready state events: http://www.w3schools.com/ajax/ajax_xmlhttprequest_onreadystatechange.asp