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You are using a array as an object, the difference between getElementbyId and
getElementsByClassName is that:

• 
  getElementbyId will return you an object.


• 
  getElementsByClassName will return you an array.

getElementsByClassName

The getElementsByClassName(classNames) method takes a string that
contains an unordered set of unique space-separated tokens
representing classes. When called, the method must return a live
NodeList object containing all the elements in the document that
have all the classes specified in that argument, having obtained the
classes by splitting a string on spaces. If there are no tokens
specified in the argument, then the method must return an empty
NodeList.

https://www.w3.org/TR/2008/WD-html5-20080610/dom.html#getelementsbyclassname

getElementById

The getElementById() method accesses the first element with the specified id.

http://www.w3schools.com/jsref/met_doc_getelementbyid.asp

in your code the lines:

1- document.getElementsByClassName('myElement').style.size = '100px';

will NOT work as expected, cos the getElementByClassName will return an array, and the array will NOT have the style property, you gonna access each element by iterating them.

That's why the function getElementById was working for you, this function will return you the direct object , and so you will be able to access the style property.

The following description is taken from this page:

The getElementsByClassName() method returns a collection of all elements in the document with the specified class name, as a NodeList object.

The NodeList object represents a collection of nodes. The nodes can be
accessed by index numbers. The index starts at 0.

Tip: You can use the length property of the NodeList object to determine the number of elements with a specified class name, then you can loop through all elements and extract the info you want.

So, as a parameter getElementsByClassName would accept a class name.

If this is your HTML body:

<div id="first" class="menuItem"></div>
<div id="second" class="menuItem"></div>
<div id="third" class="menuItem"></div>
<div id="footer"></div>

then var menuItems = document.getElementsByClassName('menuItem') would return a collection (not an array) of the 3 upper <div>s, as they match the given class name.

You can then iterate over this nodes (<div>s in this case) collection with:

for (var menuItemIndex = 0 ; menuItems.length ; menuItemIndex ++) 
 var currentMenuItem = menuItems[menuItemIndex];
 // do stuff with currentMenuItem as a node.

Please refer to this post for more on differences between elements and nodes.

ES6 provides Array.from() method, which creates a new Array instance from an array-like or iterable object.

let boxes = document.getElementsByClassName('box');

Array.from(boxes).forEach(v => v.style.background = 'green');
console.log(Array.from(boxes));

.box 
 width: 50px;
 height: 50px;
 margin: 5px;
 background: blue;
 display: inline-block;

<div class='box'></div>
<div class='box'></div>
<div class='box'></div>
<div class='box'></div>

As you can see inside the code snippet, after using Array.from() function you are then able to manipulate over each element.

The same solution using jQuery.

$('.box').css('background':'green');

.box 
 width: 50px;
 height: 50px;
 margin: 5px;
 background: blue;
 display: inline-block;

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class='box'></div>
<div class='box'></div>
<div class='box'></div>
<div class='box'></div>

In Other Words

• document.querySelector() selects only the first one element of the specified selector. So it doesn't spit out an array, it's a single value. Similar to document.getElementById() which fetches ID-elements only, since IDs have to be unique.




• document.querySelectorAll() selects all elements with the specified selector and returns them in an array. Similar to document.getElementsByClassName() for classes and document.getElementsByTagName() tags only.

Why use querySelector?

It's used merely for the sole purpose of ease and brevity.

Why use getElement/sBy?*

Faster performance.

Why this performance difference?

Both ways of selection has the purpose of creating a NodeList for further use.
querySelectors generates a static NodeList with the selectors thus it must be first created from scratch.
getElement/sBy* immediately adapts the existing live NodeList of the current DOM.

So, when to use which method it's up to you/your project/your device.

Infos

Demo of all methods
NodeList Documentation
Performance Test

It returns Array-like list.

You make that an Array as example

var el = getElementsByClassName("elem");
el = Array.prototype.slice.call(el); //this line
el[0].appendChild(otherElem); 

You could get a single element by running

document.querySelector('.myElement').style.size = '100px';

but it's going to work for the first element with class .myElement.

If you would like apply this for all elements with the class I suggest you to use

document.querySelectorAll('.myElement').forEach(function(element) 
 element.style.size = '100px';
);

/*
 * To hide all elements with the same class, 
 * use looping to reach each element with that class. 
 * In this case, looping is done recursively
 */

const hideAll = (className, i=0) => 
if(!document.getElementsByClassName(className)[i]) //exits the loop when element of that id does not exist
 return; 


document.getElementsByClassName(className)[i].style.visibility = 'hidden'; //hide element
return hideAll(className, i+1) //loop for the next element


hideAll('appBanner') //the function call requires the class name

With ES5+ (any browsed nowadays - 2017) you should be able to do

.forEach.call(document.getElementsByClassName('answer'), function(el) 
 el.style.color= 'red';
);