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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.

innerHTML not working properly when used on a jQuery object

This code isn't working as expected. It is not showing any text inside of span.day where it should be showing today's day (Tuesday at the time of writing). It is also not properly adding the class "currentDay" inside of the $.each callback.
$('#showLess span.day').innerHTML=weekday[today];
$.each($('#showMore p span.day'), function(index, item) {
if(typeof item.innerHTML != 'undefined')
{
alert('item.text:' +item.innerHTML);
alert('weekday[today]'+item.innerHTML.indexOf(weekday[today]));
if(item.innerHTML.indexOf(weekday[today])!=-1) {
alert("check which element has added class currentDay:");
item.addClass('currentDay');
}else{
if(item.hasClass('currentDay')) {
item.removeClass('currentDay');
}
}
}
});
.innerHTML is not changing the HTML, additional class is not getting added as expected.
<p id="showLess" class="less">
<span class="day">**Tuesday**</span>
</p>
Why isn't the day showing?
Why is the show/hide not working?
$('.less').on('click', function(e) {
$('#showMore').show();
$('#showLess').hide();
});
$('.more').bind('click', function(e) {
$('#showLess').show();
$('#showMore').hide();
});

You are trying to invoke JS properties on jQuery objects.
For example innerHTML
And you are trying to invoke that on a jQuery object.
$('#showLessHours span.day').innerHTML
Should be
$('#showLessHours span.day')[0].innerHTML
or
$('#showLessHours span.day').html(weekday[today]);
And in your each loop item is a JS object and you are trying to add a class using jQuery. Convert that to jQuery object first .
item.addClass('currentDay');
item.removeClass('currentDay');
should be
$(item).addClass('currentDay'); or $(this).addClass('currentDay');
$(item).removeClass('currentDay'); or $(this).removeClass('currentDay')
Instead you can use the $(this) as well instead of $(item) object inside the callback as both refer to the same objects.
Another small suggestion is why do you want to mix vanilla JS and jQuery when jQuery is included and you want to use that in your application.
jsFiddle

Because .innerHTML isn't a jquery function. You can use .html() to achieve what you are trying to do. Alternatively, if you REALLY want to use .innerHTML, you can use .get() to get the actual DOM element, and then use .innerHTML but... I wouldn't recommend it.
I believe this edited fiddle solves your problem. Relevant code:
$('#showLessHours span.day').html(weekday[today]);
//...
if(item.html() != '') {
alert('item.text:' +item.text());
alert('weekday[today]'+item.text().indexOf(weekday[today]));
if(item.html().indexOf(weekday[today])!=-1) {
//...

In addition to what Sushanth said, you are also trying to invoke jQuery methods on javascript objects.
item.addClass
should by
$(item).addClass

Add multiple elements to a single var and detect if any of those are clicked?

I have a page that can have one of three possible elements. I would like to assign whatever element exists to a var and then check if the var is clicked.
I tried using the add(), but it has confused me:
var testingVar = $('#element-one').find('.object').add('#element-two').find('.object').add('#element-three').find('.object');
$(testingVar ).click(function() {
alert('works');
});
It seems to me that the add() overwrites the previous add()? if I am on a page that has #element-three, it works, if on a page with element-one or element-two, it doesn't. If I change the var to
var testingVar = $('#element-one').find('.object');
Then a page with element-one works.
Can someone help me understand how to use the add() properly in this case?
Thanks

I think what you're looking for is this:
$('#element-one .object').add('#element-two .object').add('#element-three .object');
.find() returns a new jquery object.
However, I think this would be easier in this case:
$('#element-one .object, #element-two .object, #element-three .object');
Or even easier, if you can change markup, is to give each element you're currently selecting by id a common class, and do this:
$('.common-class .object')

jQuery load issue. Don't know how to approach this AJAX call

$("[littleBox]").load("ajax.php?eid="+$(this).attr("littlebox"));
the $(this).attr("little box") portion of the code returns undefined.
I'm trying to get the individual attribute of the initial $("[littleBox]").
this particular line of code is called as the soon as the document is ready.
when I put predefined values, such as
$("[littleBox]").load("ajax.php?eid=1");
It works as expected. Unfortunately, I need it to load specific content based on that element's attribute. Any idea how to make this work?

Loop through all items with proper this:
$("[littleBox]").each(function() {
var $this = $(this)
$this.load("ajax.php?eid="+ $this.attr("littlebox"));
});

this will not refer to $("[littleBox]") in that context, you'll have to repeat the selector - or select the element already and re-use it:
var $box = $("[littleBox]");
$box.load("ajax.php?eid=" + $box.attr("littlebox"));

post yout html that cotnain attr "little box" in it.
is it like
<a attr="little box" id="test">test<a/>
then it work like
$('#test').click(function(){
alert($(this).attr('little box'));
});

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.