Develop Reference

How do you add an element to a class (through JavaScript)?

Suppose that I want to add a newly created paragraph (using document.createElement("p")) into an existing div (with class name "container") in one of my html files. Is there a way to do this by calling some methods?
Since there's a getElementById() method, I figured I would use a getElementByClassName() method too, but that doesn't exist; what exists is getElementsByClassName() instead. One way I can get around this is to just change my div to have an id rather than a class name, and use the getElementById() to add the paragraph into the div, but I wanted to know if there was some method that I could call that would help me retrieve a class element (rather than the elements within the class itself).
I've tried looking for this online, but what I've found are answers to "how to add class names to elements" instead, which is not what I want to know.

For one element, this will chose first in DOM order:
var p = document.createElement("p");
p.innerHTML = "p element";
document.querySelector(".container").appendChild(p);
<div class="container">container</div>
For all elements with chosen class:
[...document.querySelectorAll('.container')].forEach(el => {
var p = document.createElement("p");
p.innerHTML = "p element";
el.appendChild(p);
})
<div class="container">container</div>
<div class="container">container2</div>

HTML DOM elements' IDs have to be unique within a document - and so asking for an element by Id will return you just one element (or null if there isn't a matching element).
However a class name can be applied to multiple elements, so you would expect to get zero one or more elements when searching by class, hence the getElementsByClassName returns a collection.
So if you have a list of elements with the class name container, and you know your document (hopefully) only contains one element with that name, you can pick the first element returned by the getElementsByClassName - e.g. getElementsByClassName('container')[0]
Note - getElementsByClassName returns all elements to which the class has been directly applied, for the children of the element on which it is being called. I've interpreted your query as relating to the whole document in the context of your original question.

How to hide a certain class in JavaScript

I don't know why this is not working. I am trying to hide a certain class and this is the only line of code I have on Javascript:
document.getElementsByClassName("popular").style.display = "none";
For some reason I get the error:
"document" is not defined.
What does that mean, it's not a variable.
Please help and thank you.

getElementsByClassName method returns ALL elements with the same class, think of it as an array of elements with the same class. So you need to specify, which element do you want to hide. In my example, I have only one element with such class, so I'm selecting it like the first element of an array ([0]). Your code should look like this:
document.getElementsByClassName("popular")[0].style.display = "none";
console.log('Current "display" property value is: ' + document.getElementsByClassName("popular")[0].style.display)
<div class="popular">TARGET</div>

Find element in document using HTML DOM

I need to be able to select and modify an element in an HTML document. The usual way to find an element using jQuery is by using a selector that selects by attribute, id, class or element type.
However in my case I have the element's HTML DOM and I want to find the element on my document that matches this DOM.
Important :
I know I can use a class selector or ID selector etc.. but sometimes the HTMLs I get don't have a class or an ID or an attribute to select with, So I need to be able to select from the element's HTML.
For example here is the element I need to find :
<span class='hello' data='na'>Element</span>
I tried to use jQuery's Find() but it does not work, here is the jsfiddle of the trial : https://jsfiddle.net/ndn9jtbj/
Trial :
el = jQuery("<span class='hello' data='na'>Element</span>");
jQuery("body").find(el).html("modified element");
The following code does not make any change on the element that is present in my HTML and that corresponds to the DOM I have supplied.
Is there any way to get the desired result either using native Javascript or jQuery?

You could filter it by outerHTML property if you are sure how browser had parsed it:
var $el = jQuery("body *").filter(function(){
return this.outerHTML === '<span class="hello" data="na">Element</span>';
});
$el.html("modified element");

el = jQuery('<i class="fa fa-camera"></i>');
This does not say "find the element that looks like <i class="fa fa-camera"></i>". It means "create a new i element with the two classes fa and fa-camera. It's the signature for creating new elements on the fly.
jQuery selectors look like CSS, not like HTML. To find the i element with those two classes, you need a selector like i.fa.fa-camera.
Furthermore $("document") looks for an HTML element called document. This does not exist. To select the actual document, you need $(document). You could do this:
$(document).find('i.fa.fa-camera').html("modified html")
or, more simply, you could do this:
$('i.fa.fa-camera').html('modified html');
You indicate in a comment to your question that you need to find an element based on a string of HTML that you receive. This is, to put it mildly, difficult, because, essentially, HTML ceases to exist once a browser has parsed it. It gets turned into a DOM structure. It can't just be a string search.
The best you can do is something like this:
var searchEl = jQuery('<i class="fa fa-camera"></i>');
var tagName = searchEl.prop('tagName');
var classes = [].slice.apply(searchEl.prop('classList'));
$(tagName + "." + classes.join('.')).html('modified html');
Note that this will only use the tag name and class names to find the element. If you also want IDs or something else, you'd need to add that along the same lines.

You should use Javascript getting the elements by something like
document.getElementById...
document.getElementsByClassName...
document.getElementsByTagName...
Javascript is returning the elements with the Id, Class or Tag Name you chose.

You can get en element with document.querySelector('.fa-camera')
with querySelector you can select IDs and Classes

You can simply refer to it by its class names.
$('.fa.fa-camera').html("modified html");
Similar to this answer https://stackoverflow.com/a/1041352/409556
Here is a full example:
<!DOCTYPE html>
<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
<script>
$(document).ready(function(){
$('.fa.fa-camera').html("modified html");
});
</script>
</head>
<body>
<i class="fa fa-camera"><h1>Some HTML</h1></i>
</body>
</html>`

The one thing that you could use is to check attributes (class and id goes here too in some way) that element have, and the build jQuery selector or DOM querySelector to find the element you need. The hardest part would be to find element based on innerHTML property - "Element" text inside it, for this one you'll probably have to grab all similar element and then search through them.
<span class='hello' data='na'>Element</span>
jQuery('body').find('span.hello[data=\'na\']').html('modified element')
Take notice of 'span' - that's tag selector, '.hello' - class, '[data="na"]' data attribute with name of data.
Jsfiddle link here that extends your example;

How do you grab an element relative to an Element instance with a selector?

I am writing a small library where I am in need of selecting a relative element to the targeted element through querySelector method.
For example:
HTML
<div class="target"></div>
<div class="relative"></div>
<!-- querySelector will select only this .target element -->
<div class="target"></div>
<div class="relative"></div>
<div class="target"></div>
<div class="relative"></div>
JavaScript
var target = document.querySelectorAll('.target')[1];
// Something like this which doesn't work actually
var relativeElement = target.querySelector('this + .relative');
In the above example, I am trying to select the .relative class element relative only to the .target element whose value is stored in target variable. No styles should apply to the other .relative class elements.
PS: the selectors can vary. So, I can't use JavaScript's predefined methods like previousElementSibling or nextElementSibling.
I don't need solution in jQuery or other JavaScript libraries.

Well it should be ideally:
var relativeElement = target.querySelector('.relative');
But this will actually try to select something inside the target element.
therefore this would only work if your html structure is something like:
<div class="target">
<div class="relative"></div>
</div>
Your best bet would probably in this case be to use nextElementSibling which I understand is difficult for you to use.

You cannot.
If you insist on using the querySelector of the subject element, the answers is there is no way.
The spec and MDN both says clearly that Element.querySelector must return "a descendant of the element on which it is invoked", and the object element you want does not meet this limitation.
You must go up and use other elements, e.g. document.querySelector, if you want to break out.
You can always override Element.prototype.querySelector to do your biddings, including implementing your own CSS engine that select whatever element you want in whatever syntax you want.
I didn't mention this because you will be breaking the assumption of a very important function, easily breaking other libraries and even normal code, or at best slowing them down.

target.querySelector('.relative');
By using querySelector on the target instead of document, you scope the DOM traversal to the target element.
It is not entirely clear from your explanation, but by related i assume you mean descendant?
To get all target elements you can use
document.querySelectorAll('.target')
And then iterate the result

I found a way which will work for my library.
I will replace "this " in the querySelector with a unique custom attribute value. Something like this:
Element.prototype.customQuerySelector = function(selector){
// Adding a custom attribute to refer for selector
this.setAttribute('data-unique-id', '1');
// Replace "this " string with custom attribute's value
// You can also add a unique class name instead of adding custom attribute
selector = selector.replace("this ", '[data-unique-id="1"] ');
// Get the relative element
var relativeElement = document.querySelector(selector);
// After getting the relative element, the added custom attribute is useless
// So, remove it
this.removeAttribute('data-unique-id');
// return the fetched element
return relativeElement;
}
var element = document.querySelectorAll('.target')[1];
var targetElement = element.customQuerySelector('this + .relative');
// Now, do anything with the fetched relative element
targetElement.style.color = "red";
Working Fiddle

javascript get child by id

<div onclick="test(this)">
Test
<div id="child">child</div>
</div>
I want to change the style of the child div when the parent div is clicked. How do I reference it? I would like to be able to reference it by ID as the the html in the parent div could change and the child won't be the first child etc.
function test(el){
el.childNode["child"].style.display = "none";
}
Something like that, where I can reference the child node by id and set the style of it.
Thanks.
EDIT: Point taken with IDs needing to be unique. So let me revise my question a little. I would hate to have to create unique IDs for every element that gets added to the page. The parent div is added dynamically. (sort of like a page notes system). And then there is this child div. I would like to be able to do something like this: el.getElementsByName("options").item(0).style.display = "block";
If I replace el with document, it works fine, but it doesn't to every "options" child div on the page. Whereas, I want to be able to click the parent div, and have the child div do something (like go away for example).
If I have to dynamically create a million (exaggerated) div IDs, I will, but I would rather not. Any ideas?

In modern browsers (IE8, Firefox, Chrome, Opera, Safari) you can use querySelector():
function test(el){
el.querySelector("#child").style.display = "none";
}
For older browsers (<=IE7), you would have to use some sort of library, such as Sizzle or a framework, such as jQuery, to work with selectors.
As mentioned, IDs are supposed to be unique within a document, so it's easiest to just use document.getElementById("child").

This works well:
function test(el){
el.childNodes.item("child").style.display = "none";
}
If the argument of item() function is an integer, the function will treat it as an index. If the argument is a string, then the function searches for name or ID of element.

If the child is always going to be a specific tag then you could do it like this
function test(el)
{
var children = el.getElementsByTagName('div');// any tag could be used here..
for(var i = 0; i< children.length;i++)
{
if (children[i].getAttribute('id') == 'child') // any attribute could be used here
{
// do what ever you want with the element..
// children[i] holds the element at the moment..
}
}
}

document.getElementById('child') should return you the correct element - remember that id's need to be unique across a document to make it valid anyway.
edit : see this page - ids MUST be unique.
edit edit : alternate way to solve the problem :
<div onclick="test('child1')">
Test
<div id="child1">child</div>
</div>
then you just need the test() function to look up the element by id that you passed in.

If you want to find specific child DOM element use method querySelectorAll
var $form = document.getElementById("contactFrm");
in $form variable we can search which child element we want :)
For more details about how to use querySelectorAll check this page