Your getElementById code works since IDs have to be unique and thus the function always returns exactly one element (or null if none was found).

However, the methods
getElementsByClassName,
getElementsByName,
getElementsByTagName, and
getElementsByTagNameNS
return an iterable collection of elements.

The method names provide the hint: getElement implies singular, whereas getElements implies plural.

The method querySelector also returns a single element, and querySelectorAll returns an iterable collection.

The iterable collection can either be a NodeList or an HTMLCollection.

getElementsByName and querySelectorAll are both specified to return a NodeList; the other getElementsBy* methods are specified to return an HTMLCollection, but please note that some browser versions implement this differently.

Both of these collection types don’t offer the same properties that Elements, Nodes, or similar types offer; that’s why reading style off of document.getElements…(…) fails.
In other words: a NodeList or an HTMLCollection doesn’t have a style; only an Element has a style.

These “array-like” collections are lists that contain zero or more elements, which you need to iterate over, in order to access them.
While you can iterate over them similarly to an array, note that they are different from Arrays.

In modern browsers, you can convert these iterables to a proper Array with Array.from; then you can use forEach and other Array methods, e.g. iteration methods:

Array.from(document.getElementsByClassName("myElement"))
  .forEach((element) => element.style.size = "100px");

In old browsers that don’t support Array.from or the iteration methods, you can still use Array.prototype.slice.call.
Then you can iterate over it like you would with a real array:

var elements = Array.prototype.slice
    .call(document.getElementsByClassName("myElement"));

for(var i = 0; i < elements.length; ++i){
  elements[i].style.size = "100px";
}

You can also iterate over the NodeList or HTMLCollection itself, but be aware that in most circumstances, these collections are live (MDN docs, DOM spec), i.e. they are updated as the DOM changes.
So if you insert or remove elements as you loop, make sure to not accidentally skip over some elements or create an infinite loop.
MDN documentation should always note if a method returns a live collection or a static one.

For example, a NodeList offers some iteration methods such as forEach in modern browsers:

document.querySelectorAll(".myElement")
  .forEach((element) => element.style.size = "100px");

A simple for loop can also be used:

var elements = document.getElementsByClassName("myElement");

for(var i = 0; i < elements.length; ++i){
  elements[i].style.size = "100px";
}

Aside: .childNodes yields a live NodeList and .children yields a live HTMLCollection, so these two getters also need to be handled carefully.

There are some libraries like jQuery which make DOM querying a bit shorter and create a layer of abstraction over “one element” and “a collection of elements”:

$(".myElement").css("size", "100px");

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

• getElementbyId will return an Element object or null if no element with the ID is found
• getElementsByClassName will return a live HTMLCollection, possibly of length 0 if no matching elements are found

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.

https://developer.mozilla.org/en-US/docs/Web/API/Document/getElementById

in your code the lines:

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

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

That’s why the function getElementById worked for you, this function will return the direct object. Therefore you will be able to access the style property.

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

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

setTimeout(() => {
  Array.from(boxes).forEach(v => v.style.background = 'green');
  console.log(Array.from(boxes));
}, 500);

.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>

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 ; menuItemIndex < 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.

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

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';
});

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);  

/*
 * 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 any browser supporting ES5+ (any browser basically above IE8) you can use the Array.prototype.forEach method.

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

caniuse source

So I was told that this is a duplicate from my question and I should delete mine, which I will do so I can keep the forum clean and keep the right to make questions.

As I think mine and this question are really different I will point out the answer to mine, so I will complete the knowledge in this page and the information will not be lost.

Question

1. I have a code in the snippet that has a document.getElementsByClassName("close")[0], what the [0] is doing?

2. I never seen a square brackets being used in getElementsByClassName for what purpose is it used for?

3. Also, how can I convert it to jQuery?

Answer

1. The code in the snippet has a [0] it is actually being used as a array and as it is a 0 it is referring to the first time the appointed class is being used.

2. Same thing above.

3. I couldn’t really do it and no one answered it. In the part of the code that is refering to event. target I can not use $("#myModal") instead of document.getElementById("myModal"), I think they should equivalent, but in this case the jQuery form substituting the standard one will not result in the desired effect.

    window.onclick = function(event) {
      if (event.target == modal) {
        modal.style.display = "none";
      }
    }

var modal = document.getElementById("myModal");
var btn = document.getElementById("myBtn");
var span = document.getElementsByClassName("close")[0];

btn.onclick = function() {
  modal.style.display = "block";
}
span.onclick = function() {
  modal.style.display = "none";
}
window.onclick = function(event) {
  if (event.target == modal) {
    modal.style.display = "none";
  }
}

body {font-family: Arial, Helvetica, sans-serif;}

.modal {
  display: none;
  position: fixed;
  z-index: 1;
  padding-top: 100px;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  overflow: auto;
  background-color: rgb(0,0,0);
  background-color: rgba(0,0,0,0.4);
}

.modal-content {
  background-color: #fefefe;
  margin: auto;
  padding: 20px;
  border: 1px solid #888;
  width: 80%;
}

.close {
  color: #aaaaaa;
  float: right;
  font-size: 28px;
  font-weight: bold;
}

.close:hover,
.close:focus {
  color: #000;
  text-decoration: none;
  cursor: pointer;
}

<h2>Modal </h2>

<button id="myBtn">Open Modal</button>

<div id="myModal" class="modal">
  <div class="modal-content">
    <span class="close">×</span>
    <p>Some text in the Modal..</p>
  </div>
</div>

update

It seems I can’t really delete mine question and people are unsatisfied with it, I really don’t know what I should do.

Super old school solution:

        [].forEach.call(document.getElementsByClassName('myClass'), function (el) {
            el.style.size = '100px';
        });

An answer for Drenzii’s specific case…

You could make a function that will work for any of the word elements and pass in the number of the one you want to transform, like:

// Binds `wordButtons` to an (array-like) HTMLCollection of buttons
const wordButtons = document.getElementsByClassName("word");

// Applies the `slantWord` function to the first word button
slantWord(1);

// Defines the `slantWord` function
function slantWord(wordNumber) {
  const index = wordNumber - 1; // Collection index is zero-based
  wordButtons[index].style.transform = "rotate(7deg)"; // Transforms the specified button
}

<div class="wordGameContainer">
  <button class="word word1">WORD 1</button>
  <button class="word word2">WORD 2</button>
  <button class="word word3">WORD 3</button>
  <button class="word word4">WORD 4</button>
</div>

<div>
  <button onclick="moveWord()" class="playButton">PLAY</button>
</div>