JavaScript AJAX PHP issue - php

Question

Ok. I need help with this. For some reason the onreadystatechange is fired multiple times. I really need to get this figured out tonight. It's the last task I have left and I don't know what to do or what's causing it. Please help.
I'm using AJAX (ndhr) to send over JSON 'Y-m-d h:i:s' to PHP to use the strtotime() function to return 'm-d-Y' back through AJAX. The JSON and PHP work great, but when the onreadystatechange is fired it does it multiple times. Almost like the readyState == 4 more times than it does.
var divs_d = ["d_2009", "d_2010", "d_2011"];
function ajax_get_json(cdiv,ocdv,ed){
var hr = new XMLHttpRequest();
hr.open("GET", "/json/sample.json", true);
hr.setRequestHeader("Content-type", "application/json", true);
hr.onreadystatechange = function () {
if (hr.readyState == 4 && hr.status == 200) {
cdiv.innerHTML = "";
var data = JSON.parse(hr.responseText);
var cad = data.comm_archive;
var rndate;
var nda = new Array();
var ndac = 0;
var ec = 0;
for (ni = 0; ni < cad.length; ni++) {
if (cad[ni].year == ocdv) {
ec = ec + 1;
ed.innerHTML = '<h4>' + ocdv + ' (' + ec + ' entries)</h4>';
var ndhr = new XMLHttpRequest();
var url = "/inc/strtotime.php";
var vars = "ndate=" + cad[ni].publish_date;
ndhr.open("POST", url, true);
ndhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
ndhr.onreadystatechange = function () {
if (ndhr.readyState == 4 && ndhr.status == 200) {
nda[ndac] = ndhr.responseText;
ndac = ndac + 1;
}
}
ndhr.send(vars);
}
}
nda.sort(function (a, b) { return b - a });
for (ndai = 0; ndai < ndac; ndai++) {
cdiv.innerHTML += '<h4>' + nda[ndai] + '</h4>';
}
}
}
hr.send(null);
}
function optionCchange() {
var ocdv = document.getElementById("optionCdate").value;
var ed = document.getElementById("ediv");
for (i = 0; i < divs_d.length; i++) {
var cdiv = document.getElementById(divs_d[i]);
if (divs_d[i] == "d_" + ocdv) {
cdiv.className = "bddiv show";
ajax_get_json(cdiv,ocdv,ed);
} else {
cdiv.className = "bddiv hide";
}
}
}

Answer 1

In your ndhr.onreadystatechange function ndhr represents the last ndhr created in the loop not the calling one, to reference the calling object use this.
ndhr.onreadystatechange = function () {
if (this.readyState == 4 && this.status == 200) {
nda[ndac] = this.responseText;
ndac = ndac + 1;
}
}
The the last for(ndai = 0; ndai < ndac; ndai++) is behaving as you expect because of the asynchronous nature of ajax, by the time that code is executed the ajax requests have not finished yet. You'll have to execute this code in the on ready change state callback. Just use a counter to check if all the ajax requests have finished then execute the code.

Answer 2

If you need run the code once, you don't have to be anxious about how many times readystate 4 was fired. Simply use a boolean variable to check if the block of code has been executed.
Here's a pseudocode example of my idea.
executed = false;
if (readystate && (executed == false))
{
blablabla;
executed = true;
}
else
{
sry your code has been executed;
}