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How can I reassign XHR without reloading the page in JavaScript?

This is a PHP Bootstrap CODE I try to dynamically change bootstrap modal body content that can I can easily add or edit product category . Then I submit the entered or changed data to save into database by submitting the form through Ajax .
I use pure JavaScript Ajax request to done this job for me.
For the first time when I click into Create Product Category Everything work fine. But when the first time content add then I try to add anther product the second product will automatically add two times it will increase for the forth time and so on .
I know the problem . But I can't solve it.
how can I reassign or how can I Completely clean Ajax after the task done then after clicking to create new product category I will renew Ajax ?
this is my JavaScript Ajax code :
// get add category
document.getElementById('add_category').addEventListener('click', () => {
var xhr = new XMLHttpRequest();
xhr.open("GET", 'add.php', true);
console.log("add_category")
xhr.onload = function() {
if (this.status == 200) {
document.getElementById('exampleModalLabel').innerHTML = 'Create Product Category';
document.getElementById('modal-body').innerHTML = this.response;
// console.log(this.response)
dynamicChangePicture()
// on change category_name
onChangeCategoryName('category_name');
// add new Category
document.getElementById('save_btn').addEventListener('click', (e) => {
e.preventDefault();
console.log('save_btn')
var xhr = new XMLHttpRequest();
xhr.open('POST', `../includes/functions.php`, true)
xhr.onload = function() {
if (xhr.status == 200) {
console.log(xhr.response)
if (xhr.response == 'category name add successfully! ') {
document.getElementById('close_modal_btn').click();
showMessage(xhr.response, 'Add')
console.log(xhr.responseText)
// document.getElementById('logo_box').innerHTML = `<span class='text-success'>${xhr.response}</span>`;
} else {
showMessage(xhr.response, 'Remove')
document.getElementById('logo_box').innerHTML = `<span class='text-danger'>${xhr.response}</span>`;
}
setTimeout(() => {
document.getElementById('table-body').innerHTML = '';
defaultLoad();
console.log("data Loaded")
}, 5000)
}
}
const formData = new FormData(document.getElementById('add_category_form'))
xhr.send(formData);
})
}
}
xhr.send();
})
even I Change inside xhr variable with the a deferent one it will still have the same problem.

PHP is shown on wrong page

I'm recently working on a website project. Therefor I have a website.php with all html code, a function.php and saveArray.js . In website.php I'm printing a html table with a button at the bottom. Through the button click I'm getting to the saveArray.js, where I save all the table data in an array.
With this code
var arrString = JSON.stringify(tableData);
var request = new XMLHttpRequest();
request.open('post', 'function.php', true);
request.setRequestHeader('Content-Type', 'application/x-www-form-
urlencoded');
request.send('daten=' + arrString);
I post the JS array to function.php. In function.php I do something with the array and in an if statement I want to show a modal.
The modal itself works, but I want to show it on website.php page. Which doesn't happends, because I'm currently on function.php .
How can I solve this ?
EDIT: In my array is an ID and I want to check if this ID is already in my database or not. Depending on this result I want to show the modal and upload the data if necessary. All the checking is happening in function.php

I suppose you want to inject the string returned (the modal PHP code) by your function in function.php in your current page ('website.php').
To do this, you'll have to inject the response given by the XMLHttpRequest when the request is finished.
Let's suppose we want to add all the contents within
xhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
document.getElementById("demo").innerHTML =
this.responseText;
}
};

See, You are not handling the response of the request.So handle the response.and restuern the status of the request from function.php and if data is saved the open the model. You need not go to the function.php page. See the code
var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
// this is response of the request //now check it
//Suppose you returned " data saved" as response from function.php
if(this.responseText='data saved'){
//Open model here
}
}
};
xhttp.open("POST", "function.php", true);
xhttp.send();

Data not displaying in JavaScript API

I am making a Pokedex API as a side project and I can not display the data needed to display in the different text boxes. I am using a GET request to request the height, weight, type, and ability.
<script>
$("button").click( function(){
var pokemonName = $('pokemon').val(pokemon);
event.preventDefault();
getPokemonData(pokemonName);
})
function getPokemonData(pokemonName){
var request = new XMLHttpRequest()
//GET request with link
request.open('GET','https://pokeapi.co/api/v2/pokemon/' + pokemonName, true);
// request for data
request.onload =function(){
var data = JSON.parse(this.response)
if(request.status >= 200 && request.status <= 400)
{
// outputs data
$(pokemonheight).val(response.height)
$(pokemonweight).val(response.weight)
$(pokemonAblity).val(response.ability)
$(pokemonType).val(response.type)
}
else
{
alert ("Error");
}
request.send();
}
}
</script>
</html>
I have tried setting a variable that would be equal to the response JSON element and then input that into the value of the textbox.
I do not have anything returned as expected or input displayed in the console if declared.

Issue(s)
There were a few issues with your code:
var pokemonName = $('pokemon').val(pokemon); you are setting the value of some element named pokemon (not valid) here
var data = JSON.parse(this.response); where is this.response being set? Shouldn't we be receiving response in the callback?
request.send(); is inside of the onload event, so the request never gets sent
Critiques
My main critique here is that you included a fairly large library (jQuery), and didn't utilize it to make the request. $.ajax is well documented and cleans up a lot of the intricacies of XMLHttpRequest.
The solution
$("button").click(function() {
var pokemonName = $('#pokemon').val();
//event.preventDefault();
getPokemonData(pokemonName);
})
function getPokemonData(pokemonName) {
var request = new XMLHttpRequest()
//GET request with link
request.open('GET', 'https://pokeapi.co/api/v2/pokemon/' + pokemonName, true);
// request for data
request.onload = function(response) {
var data = JSON.parse(response.currentTarget.response)
if (request.status >= 200 && request.status <= 400) {
// outputs data
console.log(data)
} else {
alert("Error");
}
}
request.send();
}
<input id="pokemon" value="12" />
<button>search</button>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
Taking all the above issues into account, I was able to get a working example of what it should ultimately look like.
Hope this helps!

XMLHttpRequest does not send file

I want to upload a file trough a XMLHttpRequest. i have looked everywhere for examples and found quite a few. But i cant figer out what it is i am doing wrong. This is my code. The function is triggerd when a button is pressed. It not wrapped in from tags
function upl_kost() {
var url = "proces_data.php?ref=upload_kost";
var hr;
var file = document.getElementById("file_kost");
var formData = new FormData();
formData.append("upload", file.files[0]);
if (window.XMLHttpRequest) {
hr=new XMLHttpRequest();
} else {
hr=new ActiveXObject("Microsoft.XMLHTTP");
}
hr.open("POST", url, true);
hr.setRequestHeader("Content-type", "multipart/form-data");
hr.onreadystatechange = function() {
if(hr.readyState == 4 && hr.status == 200) {
var return_data = hr.responseText;
alert(return_data);
}
}
hr.send(formData);
}
and this function catches it.
if($_GET['ref'] == 'upload_kost') {
var_dump($_FILES);
}
My problem is that the $_FILES stays empty. When i look at the file.files variable in the js its loaded with the data from the file that i am trying to upload.
Thanks!

Reduce your JavaScript down to minimum required for this, then add in some helpful messages you can look in your console for
function upl_kost() {
var xhr = new XMLHttpRequest(),
url = 'proces_data.php?ref=upload_kost',
fd = new FormData(),
elm = document.getElementById('file_kost');
// debug <input>
if (!elm)
console.warn('Element not found');
else if (!(elm instanceof HTMLInputElement))
console.warn('Element not an <input>');
else if (!elm.files || elm.files.length === 0)
console.warn('<input> has no files');
else
console.info('<input> looks okay');
// end debug <input>
fd.append('upload', elm.files[0]);
xhr.addEventListener('load', function () {
console.log('Response:', this.responseText);
});
xhr.open('POST', url);
xhr.send(fd);
}
If you're still having a problem, it may be server-side, e.g. are you performing a redirect before trying to access $_FILES?

Your problem is that you're setting the content type of the request
hr.setRequestHeader("Content-type", "multipart/form-data");
If you ever saw a multipart/formdata post you'll notice the content type header has a boundary
Content-Type: multipart/form-data; boundary=----webko2354645675756
which is missing from your code.
If you do not set the content type header the browser will correctly set it and the required boundary. This will allow the server to properly parse the request body.

Popup preview of a textarea using a PHP function

I'd like to make a popup preview of a textarea, using a PHP function inside the popup.
Let's say you type "Hello world" in the textarea, then you click "preview" and you get your text converted to "Hey you" by a PHP function in a popup (of course the function is not that simple, that's the reason why I can't adapt this in pure javascript).
Is it possible to do so ?
I know it could easily send the form to an intermediate page, but I must keep the form in background... that's why I need a quick preview on fly.
I did the following:
function PreviewMe() {
var newWin = window.open("", "_blank");
newWin.document.write("<html><body>"+document.getElementById('myText').value+"</body></html>");
newWin.document.close();
}
and
<textarea id="myText" ... />
<input type="submit" ... onclick="PreviewMe();">
Obviously it works without reformatting anything, so how to reformat this result in the popup please ?
Would it be possible (and mayber a better option) to use XMLHttpRequest ?
Thx !

Yes , you should use an XHR request to send data to a script which will return you data to be manipulated on the client side.

Thanks, it was by far easier in the end.
In case it might help others, here is what I've done.
Js function became :
function PreviewMe() {
var xhr = null;
if (window.XMLHttpRequest || window.ActiveXObject) {
if (window.ActiveXObject) {
try {
xhr = new ActiveXObject("Msxml2.XMLHTTP");
} catch(e) {
xhr = new ActiveXObject("Microsoft.XMLHTTP");
}
} else {
xhr = new XMLHttpRequest();
}
} else {
alert("XMLHTTPRequest not supported...");
return;
}
xhr.open("POST", "page.php", true);
xhr.setRequestHeader("Content-type","application/x-www-form-urlencoded");
xhr.onreadystatechange = function() {
if (xhr.readyState == 4 && (xhr.status == 200 || xhr.status == 0)) {
document.getElementById('show').innerHTML = xhr.responseText;
}
};
xhr.send("var="+document.getElementById('myText').value+"");
return;
}
Of course page.php includes my PHP function, show is the id of the div where the result is printed.