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JavaScript AJAX Basics (retiring) AJAX Concepts GET and POST

Chris Reich
Chris Reich
15,163 Points
Posted by Chris Reich
Chris Reich
15,163 Points

What does xhr.readyState === 4 mean in the code below? Why 4?

var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function () {
      if (xhr.readyState === 4) {
        document.getElementById('ajax').innerHTML = xhr.responseText;
      }
    };
    xhr.open('GET', 'sidebar.html');
    function sendAJAX() {
    xhr.send();
      document.getElementById('load').style.display = "none";
    }

4 Answers

jason chan
jason chan
31,009 Points
jason chan
jason chan
31,009 Points 
readyState  Holds the status of the XMLHttpRequest. Changes from 0 to 4: 
0: request not initialized 
1: server connection established
2: request received 
3: processing request 
4: request finished and response is ready
Abhishek Bhardwaj
Abhishek Bhardwaj
3,316 Points
Abhishek Bhardwaj
Abhishek Bhardwaj
3,316 Points

As Jason chan wrote request check the condion from 0 to 4 and 4th states define the download state means 'request finished and response is ready' readyState Holds the status of the XMLHttpRequest. Changes from 0 to 4: 0: request not initialized 1: server connection established 2: request received 3: processing request 4: request finished and response is ready

David Weigel
David Weigel
8,265 Points
David Weigel
David Weigel
8,265 Points

To add to the information from Jason - a readyState of 4 means that the program has "heard" back from the server. You want to wait for this (hence the if statement) before executing anything further.
Full documention from MDN: https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest

NiKole Maxwell
NiKole Maxwell
11,083 Points
NiKole Maxwell
NiKole Maxwell
11,083 Points

I actually found the info page on W3 useful for this question as well. https://www.w3schools.com/xml/ajax_xmlhttprequest_create.asp