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Using a AJAX, how do I set the innerHTML of an element that is not yet loaded in the DOM?

There is a main-feed showing a list of blog post titles. When a title is clicked, I need to display the details of the blog post in a new html file. Below is my current code:
window.location.href = "/viewpost.html";
postID = this.id;
if (window.XMLHttpRequest) {
// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp = new XMLHttpRequest();
} else {
// code for IE6, IE5
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
document.getElementById("view-post-container").innerHTML = xmlhttp.responseText;
}
};
xmlhttp.open("GET", "viewpost.php?q=" + postID, true);
xmlhttp.send();
The problem is, the element view-post-container, is in the file viewpost.html, and so I don't think the PHP file can access it. I would just display the data on the current page (index.php), but I wanted to have individual pages for each post so I can eventually learn how to have dynamic URL's for SEO and sharing purposes. The end goal is having dynamic urls, so maybe there is a better approach? Any help would is much appreciated. Thanks.

just try this, You have to put your code in on window.onload function
window.onload = function() {
if (window.XMLHttpRequest) {
xmlhttp = new XMLHttpRequest();
} else {
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
document.getElementById("view-post-container").innerHTML = xmlhttp.responseText;
}
};
xmlhttp.open("GET", "viewpost.php?q=" + postID, true);
xmlhttp.send();
}

PHP closes program when using AJAX

I have a JS function that calls a PHP code using AJAX:
function switchCameraStatus() {
if (window.XMLHttpRequest) {
// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp = new XMLHttpRequest();
} else {
// code for IE6, IE5
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
document.getElementById("lastChange").innerHTML = xmlhttp.responseText;
} else {
document.getElementById("lastChange").innerHTML = "Error switchCameraDiv";
}
}
xmlhttp.open("GET","camera-control.php?function=switchCamera()", false);
xmlhttp.send();
}
The function called in PHP is:
function enableCamera() {
$command = '../scripts/videoOn.sh &';
exec($command);
echo "Camera enable";
}
The problem is that the camera turns off automatically in less than a second that is the same to say the process exits.
Edited: It stops when executing the .php on a browser, but It works on bash with the php file.php command. I tried with nohup, but it stops on the browser execution.

pass text box value from one page to another page without page refresh?

i have two text box i am trying to pass that textbox value from one page to another page with out page refresh i am using ajax for this but i am able to achive only one text box value
Here is my code
<script>
function loadXMLDoc() {
var xmlhttp;
if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp = new XMLHttpRequest();
} else {// code for IE6, IE5
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
document.getElementById("myDiv").innerHTML = xmlhttp.responseText;
}
}
xmlhttp.open("GET", "testing.php?message="+ document.getElementById("message").value+"service="+document.getElementById("city").value, true);
xmlhttp.send();
}
//return false;
</script>
i think i am wrong here in this line
xmlhttp.open("GET", "testing.php?message="+ document.getElementById("message").value+"service="+document.getElementById("city").value, true);
How can i achieve my goal
Any help will be apprecieted

Your query string pairs need to be seperated with an ampersand:
xmlhttp.open("GET", "testing.php?message="+ document.getElementById("message").value+"&service="+document.getElementById("city").value, true);
//Here -----^

AJAX function( ) part alone not working

After function() it is not working, i don't know why. If I put an alert before that statement it's working but after that statement it isn't working.
<script>
function new_order() {
var xmlhttp;
if (window.XMLHttpRequest) {
xmlhttp = new XMLHttpRequest();
}
alert("asdasd");
xmlhttp.onreadystatechange = function () {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
document.getElementById("order_id").innerHTML = xmlhttp.responseText;
}
}
xmlhttp.open("POST", "item_sort.php?sort=" + str, true);
xmlhttp.send();
}
</script>

3 things you can check
If an element corresponding to id order_id exists on the page
if the str is not null or not defined
If you are using older IE versions IE5 or 6 you need to add the
following in your code.
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
Also you need to use the following way if you want to do POST ajax call.
xmlhttp.open("POST", "item_sort.php", true);
xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
xmlhttp.send("sort=" + str);

Using JSON to parse xmlhttp.responseText for populating textboxes

How can JSON be used to parse xmlhttp.responseText? I can't seem to get textboxes populated using the parsed data. I tried using .value and .innerHTML with the dot notation with b.first and b.second used with json_encode from the loadTextBox.php file (see below), but the textboxes won't populate.
Main page code:
function loadDoc()
{
var xmlhttp;
// code for IE7+, Firefox, Chrome, Opera, Safari
if (window.XMLHttpRequest)
{
xmlhttp=new XMLHttpRequest();
}
//code for IE6, IE5
else
{
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
var doc = window.document.createElement("doc");
var a = xmlhttp.responseText;
var b = JSON.parse(a);
document.getElementById("textbox").innerHTML=b.first;
document.getElementById("textbox2").innerHTML=b.second;
}
}
xmlhttp.open("GET","loadTextBox.php?id=4",true);
xmlhttp.send();
}
loadTextBox.php code:
<?php
---Placeholder for correct DB login info---
$result = $mysql->query("SELECT column_one FROM table_one");
while ($row = $result->fetch_object())
{
$queryResult[] = $row->present_tense;
}
$textboxValue = $queryResult[0];
$textboxValue2 = $queryResult[2];
echo json_encode(array('first'=>$textboxValue,'second'=>$textboxValue2));
?>

This is fully tested and works. Use as a starting point to accomplish what you are trying to do:
var url = "YOUR.php"
var ajax = new XMLHttpRequest();
ajax.open("GET", url, true);
ajax.send(null);
ajax.onreadystatechange = function () {
if (ajax.readyState == 4 && (ajax.status == 200)) {
console.log("ready")
var Data = JSON.parse(ajax.responseText);
console.log(Data);
console.log(Data.first);
} else {
console.log("not ready yet")
}
}
This assumes your JSON output is properly formatted as you stated:
{"first":"radim","second":"radi"}

I have figured out the underlying problem. Extra tags were being sent because I had unnecessary tags in my DB info file. Those tags were being sent in the responseText with {"first":"radim","second":"radi"}. So the code pertaining to ---Placeholder for correct DB login info--- was wrong. I also changed .innerHTML to .value and now it works as intended.
Main page code updated:
function loadDoc()
{
var xmlhttp;
// code for IE7+, Firefox, Chrome, Opera, Safari
if (window.XMLHttpRequest)
{
xmlhttp=new XMLHttpRequest();
}
// code for IE6, IE5
else
{
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
var a = JSON.parse(xmlhttp.responseText);
document.getElementById("textbox").value=a.first;
document.getElementById("textbox2").value=a.second;
}
}
xmlhttp.open("GET","loadTextBox.php?id=4",true);
xmlhttp.send();
}