Develop Reference

textContent returns undefined

I have a table in which I want to extract the text of the active item. I do this with the following code:
var addedWorkout = $("#custDropDownMenuA").find(".dropdown-item.active");
console.log(addedWorkout);
addedWorkout = addedWorkout.textContent;
console.log(addedWorkout);
The problem is that I keep getting undefined. I checked the console and it indeed finds the element I want without fail.
I am relatively new to Javascript, but after over an hour of Googling I could not find the issue and I don't understand why. I know that I can get the text element if I hardcore it using the following line:
document.querySelector("#selectiona1").textContent
but not with:
$("#selectiona1").textContent
What is the difference between these 2? I read that textContent is part of the DOM, to my understanding it relates to objects and according to my console i think it is an object. I made some crazy attempts like putting the object I got into the querySelector, but nothing works.

With this line:
var addedWorkout = $("#custDropDownMenuA").find(".dropdown-item.active");
you're using jQuery to select the .dropdown-item.active inside #custDropDownMenuA, and when you select with jQuery, you get a jQuery object in response. So, addedWorkout is a jQuery object, and jQuery objects generally do not have the same properties/methods as standard HTMLElements. (querySelector is the vanilla Javascript method to retrieve an element)
Either select the [0]th item in the jQuery collection to get to the first matching element:
var addedWorkout = $("#custDropDownMenuA").find(".dropdown-item.active")[0];
Or use the jQuery method to get the text of the first matching element, which is .text():
var addedWorkoutText = addedWorkout.text();
(note the use of a new variable - you will likely find it easier to read and debug code when you create new variables rather than reassigning old ones, when possible)

Your var 'addedWorkout' is a Jquery object, not a html element.
To show the text use:
addedWorkout.text();
Alternatively, you can change the 'addedWorkout' to a html element by adding the index [0], like this:
addedWorkout[0].textContent;

Using variables with jQuery's replaceWith() method

Ok this one seems pretty simple (and it probably is). I am trying to use jQuery's replace with method but I don't feel like putting all of the html that will be replacing the html on the page into the method itself (its like 60 lines of HTML). So I want to put the html that will be the replacement in a variable named qOneSmall like so
var qOneSmall = qOneSmall.html('..........all the html');
but when I try this I get this error back
Uncaught SyntaxError: Unexpected token ILLEGAL
I don't see any reserved words in there..? Any help would be appreciated.

I think the solution is to only grab the element on the page you're interested in. You say you have like 60 lines. If you know exactly what you want to replace..place just that text in a div with an id='mySpecialText'. Then use jQuery to find and replace just that.
var replacementText = "....all the HTML";
$("#mySpecialText").text(replacementText);
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="mySpecialText">Foo</div>

If you're only looking to replace text then jaj.laney's .text() approach can be used. However, that will not render the string as HTML.
The reason the way you're using .html() is likely illegal is that qSmallOne is not a JQuery object. The method cannot be performed on arbitrary variables. You can set the HTML string to a variable and pass that string to the .html() function like this:
var htmlstring = '<em>emphasis</em> and <strong>strong</strong>';
$('#target').html(htmlstring);
To see the difference between using .html() and .text() you can check out this short fiddle.
Edit after seeing the HTML
So there is a lot going on here. I'm just going to group these things into a list of issues
The HTML Strings
So I actually learned something here. Using the carriage return and tab keys in the HTML string is breaking the string. The illegal-ness is coming from the fact the string is never properly terminated since it thinks it ends at the first line. Strip out the white space in your strings and they're perfectly valid.
Variable Names
Minor thing, you've got a typo in qSmallOne. Be sure to check your spelling especially when working with these giant variables. A little diligence up front will save a bunch of headache later.
Selecting the Right Target
Your targets for the change in content are IDs that are in the strings in your variables and not in the actual DOM. While it looks like you're handling this, I found it rather confusing. I would use one containing element with a static ID and target that instead (that way you don't have to remember why you're handling multiple IDs for one container in the future).
Using replaceWith() and html()
.replaceWith() is used to replace an element with something else. This includes the element that is being targeted, so you need to be very aware of what you're wanting to replace. .html() may be a better way to go since it replaces the content within the target, not including the target itself.
I've made these updates and forked your fiddle here.

object HTMLcollection[0] keeps returning undefined

Suppose we have something like:
1
2
3
When I try something like
alert(document.getElementsByClassName("my-list"))
I get object HTMLCollection. And if I try something like alert(document.getElementsByClassName("my-list")[0]) I get undefined. How can I get the first href in the list? So it would be "1" in this case.

Check this in Fiddler. Place the document.getElementsByClassName("my-list") in a round bracket and the add the index [0] to it.
**UPDATE**: Use `window.onload` to perform operations after all DOM elements
are loaded.
window.onload = function()
{
alert((document.getElementsByClassName("my-list"))[0])
}
1
2
3

It could happen if you add the script in the HTML header. In this case, you saw HTMLCollection; however, the items would be empty.
move the script to the bottom of the HTML body.

Also, just be sure to check your code, I had this problem from using querySelector. If you use querySelector it only returns one element (which will return undefined when iterating), while querySelectorAll creates a html collection which you can iterate through.

Loop though DOM with jQuery to get some data- attribute value

This seems like a simple thing, but I keep getting "undefined"
I am trying out the "data-" HTML5 attribute and I am looping through a bunch of div tags that look like this:
<div id="myEvent"
data-scheduledOn="1399985100000"
data-eventStatus="3">
And I am looping through a bunch of these like this:
$('[id="myEvent"]').each(function(index, divItem) {
alert($(divItem).data("scheduledOn"));
}
But I keep getting "undefined" If I do this (get the attribute) it works fine:
alert($(divItem).attr("data-scheduledOn"));
So What am I missing?

http://api.jquery.com/data/
"The .data() method allows us to attach data of any type to DOM elements in a way that is safe from circular references and therefore from memory leaks."
At least at this point in time, to use the .data function you have to attach the data using the function before you can read it back using the .data function.
If you need to read pre-existing data use the .attr or .prop functions.

It seems as though It is a naming problem as Hamza Kubba suggested, but just a bit different...
if I changed the name of the data attribute to "data-scheduled-on" I can retrieve it by .data("scheduledOn") OR using data-scheduledon and .data("scheduledon") also works.
So don't use CAPS for data- names is the moral of this story!

Please note that per HTML 5 specs, the attribute name should not contain any uppercase letters and some browsers such as FF & Chrome will change any uppercase letter to lowercase. That's why the following demo works if you access the data attributes with lowercase names:
http://jsfiddle.net/fiddleyetu/5LdQd/
$('div.myEvent').each(function(index, divItem) {
console.log($(divItem).data("scheduledon"));
console.log( $(divItem).data("eventstatus") );
});
Ans since you cannot have more than one element on a page with the same ID, I have used a class selector for the demo.
MORAL: Do not use UPPERcase; your browsers may not always be that 'understanding'.

Outputting String to HTML using javaScript

there have been many questions like this before, but the answer usually involved referencing the element to be outputted to within the script. in my case, elements are created dynamically on load (it's hosted on Tumblr, to be specific), so there's no referable ID for each of the elements I need to output a string to.
is it possible to do this? jQuery can be used if it makes things much simpler. my function takes a string as an input and outputs a string using return.

$('.post .posttext a').html(YourfunctionThatReturnsAString());
This should do it

$('.post .posttext a') -> this returns an array of a elements.
$('.post').eq(the post number).find('.posttext a') -> replace "the post number" with the number of your post if you want to target something more specific.
You can then call the text("your text") method to set the text.