How to work with 2 XMLHttpRequest one dependent on another?

Question

I am working on a project where I have got 2 XMLHttpRequest() objects, say A and B.

What I want to accomplish is when A finish fetching a list of data items, B will be triggered to fetch some more items based on the previous data items fetch by A.

Currently my problem is that the two objects are working independent of one another.

My code is below:

            var A = new XMLHttpRequest();

            var B = new XMLHttpRequest();

            A.open("GET", directory, true);
            A.onreadystatechange = function () {

                if (A.readyState === 4) {
                    if (A.status === 200 || A.status == 0) {
                     //does... something
                     }
                }

            }
            A.send(null);
            while(true){

                B.open("GET", another_directory, false);
                B.overrideMimeType("application/document");
                B.send(null);
                if (B.status == "404")
                continue;

                 //does... something else
            }

This code is not working because I find evertime B proceed before A can complete. I basically don't know which event to use.

How can I accomplish my objective? What events can I use so that I can sync processing B right after finishing with A?

Answer 1

(After lengthy edit) I'd recommend strongly that you take the time to understand the nature of asynchronous calls within JavaScript. Here's a bit of recommended reading.Asynchronous Programming in JavaScript I think that is simple enough to understand what is going on. Note: Stop reading at "Enter Mobl".

In JavaScript when you call a function, the system places that function into a 'queue' with an implicit instruction to go ahead and run it as soon as you can. It does that for each and every function call. In your case you are telling the system to run A, then run B. A goes in the queue, B goes in the queue. They are submitted as individual functions. B happens to run first.

For normal functions, if you want to control the sequence, you can nest the A function call within the B function call. But oops. You are using XMLHttpRequest, so that limits your ability to customize the functions. Read on. Check out Ajax Patterns on the subject Look at the paragraph for "Asynchronous Calls". Look at your code...

A.onreadystatechange = function () {
    if (A.readyState === 4) {
         if (A.status === 200 || A.status == 0) {
             //does... something
             (RUN ALL THE B.methods right here...)
         }
    }
}

I think that will get you to your destination, assuming you want a no jQuery solution.

For the person who just wants a functioning system, and doesn't want to understand the language better, here is a jquery solution... Note how the B function call is nested within the A function call. Do note that the order of this nesting is based on the presence of the jQuery success tag. If not using jQuery, you will manually have to nest the functions as appropriate.

var special_value;
$("button").click(function(){
    $.ajax({url: "demo_testA.html", 
            type: 'GET',
            success: function(resultA){
               special_value = resultA;
               $.ajax({url: "demo_testB.html",
                      type: 'GET', 
                      data: special_value, 
                      success: function(resultB){
                            $("#div1").html(resultB);
               }});
    });
});

I will say, it would be much easier to help you help yourself with the use of better communications. If you don't like something, then so state. If you don't understand something ask for more clarification or edit your problem statement. Feedback is a good thing.

Answer 2

Here is an alternative solution, either use the fetch API or promisify native XHR and this problem becomes much simpler:

fetch(directory).then(function(response){
    // some processing
    return fetch(another_directory); // can change content type too, see the mdn docs
}).then(function(responseTwo){
      // all processing is done
}).catch(function(err){
      // handle errors from all the steps above at once
});

This is just as native as XHR, and is much much simpler to manage with promises.