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Document.querySelector returns a NULL value - script is at bottom of page

I am trying to get the patientNumber (ClinicA100-PF-TR1-P1) using querySelector. I keep getting a NULL value. The patientNumber is at the top of the page and the script is at the bottom. Even after the page is loaded, I click a button that runs the function and it still returns a NULL value.
Here is a screenshot of the selectors (https://recordit.co/IypXuuXib0)
<script type="text/javascript">
function getPatientNumber(){
var patientNumber = document.querySelector("patientNumber");
console.log(patientNumber);
console.log("hello");
return patientNumber;
}
var patientNumber = getPatientNumber();
console.log(patientNumber);
_kmq.push(['identify', patientNumber]);
</script>
Thank you for any help you can provide.
ADDITIONAL HTML INFORMATION:
I am using Caspio (database management software) to create this HTML code. I don't know if that may be the cause of the issue. Here is the HTML CODE.
<p class="sponsorName" id="sponsorNameID">[#authfield:User_List_Sponsor_Name]</p>
<p class="clinicNumber" id="clinicNumberID">[#authfield:User_List_Site_Number]</p>
<p class="protocolNumber" id="protocolNumberID">[#authfield:User_List_Protocol_Number]</p>
<p class="patientNumber" id="patientNumberID">[#authfield:User_List_Patient_Number]</p>

You are missing a dot.
var patientNumberNode = document.querySelector(".patientNumber");
var patientNumber = patientNumberNode.innerText;

if you select the item with class".", if you select with id, you should use"#".
var patientNumber = document.querySelector(".patientNumber"); // class select
var patientNumber = document.querySelector("#patientNumber"); // id select

Your selector is incorrect. It should be
var patientNumber = document.querySelector(".patientNumber");
Why is it failing:
When you use patientNumber as the selector, JavaScript looks for an element with a name of patientNumber. Since that's not the case, and you are looking for an element with a class of patientNumber, you need to use the . notation.
Addon Suggestion (can be ignored):
Since you are also using IDs, consider using document.getElementById() as it is faster than using document.querySelector().
Note that if you use document.getElementById(), your .patientNumber selector won't work. You need to write it as
document.getElementById('patientNumberID');
//ID based on the screenshot of the DOM you've shared

While the code is at the bottom of the page, and the element is at the top, it is not loaded asynchronously as it comes from a third party database. i put a delay in the getPatientNumber() and it works now.

HTML button tag removal from backend

I have created a button dynamically in JavaScript and i am deleting the same when user clicks X symbol on it..
I am setting style.display=none to achieve this but the problem is in HTML there is space in it even after deleting the button. How can i delete the button without any space in HTML.
var x = document.getElementById(id);
x.style.display='none';

First of all, if you Google remove HTML element in javascript the first thing you see is this: How to remove an HTML element using Javascript?
and it works, but I'll pitch in my 2 cents anyways since that one uses parentElement, which is kinda outdated.
There's a .remove() method in javascript that lets you remove an html element.
It's as simple as:
var x = document.querySelector("#id");
x.remove();
but I doubt that will solve your issue, you might want to remove the parent element if it has a margin or padding like so:
var x = document.querySelector("#id");
x.parentElement.remove();
cheers

try it if u want to change the css
<script type="text/javascript">
$("#id").css('display', 'none');
</script>
or use it for add or remove the class. change the add to remove if want remove the class
<script type="text/javascript">
document.getElementById("submit").classList.add('disabled');
</script>

Get number of table under a div tag

Suppose I have unknown number of tables under a div
How can I count many table there are under that div in either plain JavaScript or jQuery?

var particularDiv = document.getElementById('particularDiv');
var allTables = particularDiv.getElementsByTagName('table').length;
You can get the particular div and get table tags under that div by using pure javascript like above.

You can use length:
var numberOfTables = $('div#yourDivId table').length;

try:
$("table", "#your-div-id").length;

sure, if you are willing to use jquery:
$("#yourdiv").find("table").length

This would work:
var tableCount = $('#myDiv table').length;

Extracting text from a HTML to be stored as a JS variable, then to be added to a separate HTML's element

Alrite, I have seen other Questions with similar titles but they don't do exactly what Im asking.
I have 2 x HTML documents, one containing my page, one containing a element with a paragraph of text in it. As-well as a separate .js file
what I want to do is extract this text, store it as a JS variable and then use jQuery to edit the contents of an element within the main page. This is the conclusion I came to but it didnt work as expected, im not sure if it is me making a syntax error or if i am using the wrong code completely:
$(document).ready(function(){
var c1=(#homec.substring(0))
// #homec is the container of the text i need
$(".nav_btn #1").click(function(c1){
$(".pcontent span p") .html(+c1)}
);
});
i know +c1 is most probably wrong, but i have been struggling to find the syntax on this one. thankyou in advance :D

var c1=(#homec.substring(0)) will throw an error because #homec is not a valid variable name, is undefined, and does not have a property function called substring. To get the html of an element with an id of homec, use the html method:
var c1 = $("#homec").html();
c1 should not be an argument of the click function because it is defined in the parent scope. +c1 is unnecessary because you do not need to coerce c1 to a number.
If you are trying to add content to the end of the paragraph, use the append method:
$(".pcontent span p").append(c1)
That means you should use this code instead:
$(document).ready(function() {
var c1 = $("#homec").html();
$(".nav_btn #1").click(function() {
$(".pcontent span p").append(c1)
});
});
P.S. Numbers are not valid ID attributes in HTML. Browsers support it, so it won't make anything go awry, but your pages won't validate.

Try this:
$(".nav_btn #1").click(function(c1){
var para = $(".pcontent span p");
para.html(para.html() + c1);
});

The JQuery text() function will allow you to get the combined text contents of each element in the set of matched elements, including their descendants. You can then use the text(value) function to set the text content of your target paragraph element. Something like this should suffice:
$(document).ready(function() {
var c1 = $("homec").text();
$(".nav_btn #1").click(function() {
$(".pcontent span p").text(c1);
});
});
See the JQuery documentation for more details on the text() function. If you need to capture the full structure of the other document, then try the html() function instead.

jQuery create element initially hidden

Is there a way to create an element and have it hidden initially? I am creating an iframe but don't want it shown right away.

Very simple. Just do this:
var myFrame = $("<iframe>").hide();

var my_iframe = $('<iframe name="your_iframe" src="your_source"></iframe>');
now my_iframe holds your jQuery created iframe. Modify it, do what you wish and then put it in the dom.
It wont be visible until you insert it into the dom.

Just create it and style it as being hidden, in one of many possible ways, like this:
var secretThing = $('<iframe></iframe>', { css: { 'display': 'none' }});
$('body').append(secretThing);
Another way to make something hidden is to position it far off the viewport, or to put it behind something else, or to set some dimension to zero. It depends on the rest of your design. Personally, I'd be inclined to give the element a class value that makes it hidden.
(#gilly3 wisely notes that the handy jQuery "hide" function might be a simple way to do this.)

var theElement = <create the iframe here>;
theElement.hide();
// append theElement here

Like this:
var FName = $("<iframe>").hide();