AJAX XHR request onReadyStateChange events order and number of times clarification

Question

I'm learning and trying to write a simple stock quote tool using Python-Flask and Javascript.

I specifically want to learn plain Javascript. My code is working, but what I don't understand is when I'm watching the developer console, I get 3 error messages printed before I get the successful console.log(response).

Is it simply that the code loops 3 times before the response comes back, so it logged 'ERROR' each of those times before finally returning the 200 status? Would someone explain it to me or point me to a good article/post?

My event listener:

document.getElementById("btn_quote").addEventListener("click", getQuote);

The ajax call:

function getQuote(e){
    e.preventDefault();
    var ticker = document.getElementById("ticker").value
    var shares = document.getElementById("shares").value
    var url = "/quote/"+ticker+"/"+shares

    // Fetch the latest data.
    var request = new XMLHttpRequest();
    request.onreadystatechange = function() {
        if (request.readyState === XMLHttpRequest.DONE) {
            if (request.status === 200) {
                var response = JSON.parse(request.response);
                console.log(response);
            }
        } else {
            // TODO, handle error when no data is available.
            console.log('ERROR');
            return false;
        }
    };
        request.open('GET', url);
        request.send();
} 

Answer 1

It's not returning separate HTTP status codes, its returning different ready states.

Change your console.log("ERROR"); . To console.log(request.readyState); .

Then you will see what it is reporting and why.

Answer 2

i think you should be checking your readyState values with the actual values of the response. For you reference, following are the possible values of readyState :

0: request not initialized 
1: server connection established
2: request received 
3: processing request 
4: request finished and response is ready

So you could basically check it to be 4 in your case:

var request = new XMLHttpRequest();
    request.onreadystatechange = function() {
        if (request.readyState === 4) {
            //response statements
        } else {
            //error statements
        }
    };

Answer 3

Basically, ajax calls will get notified for the following events which is called as readyStateChange event.

For most cases, you used to get 4 ready state changes based on the speed of the connection (rare cases only only one if it's very quick) and you should check whether it is 4 which means the response is completed now.

To check whether the request is suceess or not, you should check the request.status === 200 which means success and can check for other http status code for errors like 404 , 500 etc.

 document.getElementById("btn_quote").addEventListener("click", getQuote); document.getElementById("btn_quote_error").addEventListener("click", getQuoteError); function getQuote(e){ e.preventDefault(); var ticker = document.getElementById("ticker").value; var shares = document.getElementById("shares").value; //var url = "/quote/" + ticker + "/" + shares; var url = 'http://stackoverflow.com/'; // Fetch the latest data. var request = new XMLHttpRequest(); request.onreadystatechange = function() { console.log(request.readyState); if (request.readyState === XMLHttpRequest.DONE) { console.log(request.status); if (request.status === 200) { //var response = JSON.parse(request.response); //console.log(response); } } //else { // TODO, handle error when no data is available. //console.log('ERROR'); //return false; //} }; request.open('GET', url, true); request.send(); } function getQuoteError(e){ e.preventDefault(); var ticker = document.getElementById("ticker").value; var shares = document.getElementById("shares").value; //var url = "/quote/" + ticker + "/" + shares; var url = 'http://stackoverflow404.com/'; // Fetch the latest data. var request = new XMLHttpRequest(); request.onreadystatechange = function() { console.log(request.readyState); if (request.readyState === XMLHttpRequest.DONE) { console.log(request.status); if (request.status === 200) { //var response = JSON.parse(request.response); //console.log(response); } } //else { // TODO, handle error when no data is available. //console.log('ERROR'); //return false; //} }; request.open('GET', url, true); request.send(); } 

 <input type="text" id="ticker"/> <input type="text" id="shares"/> <input type="button" id="btn_quote" value="Get Quote" /> <input type="button" id="btn_quote_error" value="Get Quote Error" />