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What is DOM: Document Object Model

Beyond the Console!
The Website DOM
How to Update Your Website DOM
The DOM is Spectacular!
How Do I Access Any Object in the DOM?
Accessing the Element’s Children
Adding Elements to the Document
innerHTML
Removing Elements from the Document
Changing Attributes
Changing the Styles
Changing Classes
Further reading

Beyond the Console!

Nobody likes applications that are based on the console... Can you imagine using Uber from the command line?

1$ "uber" request-trip --from home-- to work --pool

Thankfully, we have browsers! They let us render our application in a visual interface that we call a website.

The Website DOM

As you already know, the responsibility of the browser is to transform HTML/CSS code into visual elements. Those elements are mapped into a hierarchy that is stored in RAM memory, and it’s called The DOM.

With JavaScript we can manipulate the DOM (website elements) during runtime (the application's lifecycle).

NOTE: Please always remember that all JavaScript code that you write in your HTML document MUST be wrapped inside a <script> tag, like this:

1<script  type="text/javascript">
2     // Your code here  
3</script>

Nowadays, we tend to write all of our JS code in a separate file with extension .js, in which case the script tag in your html's body needs to look like this:

1<body>
2    <div>some content</div>
3    <div>more content</div>
4
5    <script src="index.js" type="text/javascript"></script>
6</body>

This is the preferred way for you to write and link your JS code from now on, as well.

How to Update Your Website DOM

There are several ways to manipulate the DOM, but the most simple one is document.write(). Every time you create a document.write() you will be writing onto the HTML whatever string you decide to pass as a parameter to the write function.

It does not matter where you write the code. The only thing that matters is that it is linked to the HTML through a <script> tag. For example:

Click here to open demo in a new window

The DOM is Spectacular!

From the moment a website starts being loaded, the browser creates a hierarchy that is called The DOM. Each HTML element you coded in your HTML document as a developer has a place in that hierarchy, and you can access it using JavaScript anytime you want during the runtime.

  • Every HTML element has an object in the document hierarchy.
  • The DOM is generated during runtime.
  • Every browser tries to replicate the DOM in the exact same way, but there are some differences between them. That is why some things work in one browser, but not in others.
  • JavaScript is the only language capable of accessing The DOM during runtime.
  • The "Google Inspector" is the best representation of The DOM today.

Click here to open video in a new window

How Do I Access Any Object in the DOM?

Just like we did with CSS, we can select any element in the document. There are various methods that allow us to target the element that we want. The most current methods are:

document.querySelector("#some-id")

Returns an instance of the first element which matches the CSS selector that you specified.

document.querySelectorAll(".some-class")

Returns a list (similar to JavaScript array) with all elements which match the CSS selector.

It is preferable that you use these two methods in your work.

Other methods which are now considered deprecated but you may see in some code examples:

document.getElementById("elementId")

Returns an instance of the element with the id="elementId" in the HTML document.

document.getElementsByClassName("exampleClass")

Returns an array of all elements with the class="exampleClass" in their HTML tag property.

document.getElementsByTagName("p")

Returns an array with all the instances representing each paragraph element in the HTML document.

document.getElementsByName("name_value")

Returns an array with all the elements that have name="name_value" in the name property of their HTML tag in the HTML document.

1let elem = document.getElementById("xyz");
2elem.style.color = "red";   // change color to red  
3
4let myList = document.getElementsByTagName("p");
5let howManyElements = myList.length;
6myList[0].style.color = "red";   // make the first one red  
7
8let myList = document.getElementsByClassName("abc");
9myList[0].style.color = "red";   // make the first one red  
10
11let xyz = document.getElementsByName("xyz");
12xyz[0].style.color = "red";   // make the first one red

Accessing the Element’s Children

It is very common to need to change an element’s child. For example:

  • Update all the <li> children of a specific <ul> to make their background red.
  • Remove the first <tr> row of a <table>.
  • Hiding all the children with a specific class.
  • And the list goes on!

The best way to retrieve the children elements of any DOM element is by using its .children property, like this:

element.children

This returns an array with all the element’s children nodes.

Given the following example, open the code in JSFiddle and console.log tableElm.children at the end of the existing JavaScript code:

Click here to open demo in a new window

Open the Console in the Chrome Inspector feature. What did you observe?

You should have gotten an array (HTMLCollection) with two items - the thead and tbody which are the table's direct children. Because you see that tableElm.children gives you an array-like structure in JavaScript, you could guess that to get the first element (thead), you can simply code tableElm.children[0].

Now go to the end of the JS code and add another console.log - tableElm.children[1].children.

Can you guess what this will return in the console? Try it out!

Here is an explanation of the other properties used in the example:

1x.querySelector(".random").style.background = "green";
2// get the first #myDIV child with the .random class  
3
4x.querySelector("h3,h2").style.background = "blue";
5// get the first #myDIV child with the tag <h3> or <h2>
6 
7let tableElm = document.querySelector("#people");
8let trArray = tableElm.querySelectorAll("tr");
9trArray[3].style.background = "red";
10// get an array with all elements nested within the #people table, of type <tr> - not necessarily direct children

Adding Elements to the Document

There are 2 functions we can use for that: appendChild() and insertBefore().

Let's say that you have selected a <div> with the id="myFirstDiv" and you want to add a new <h1> inside of that <div>.

You can use the appendChild function like this:

1let divElem = document.querySelector("myFirstDiv");
2let myNewHOne = document.createElement("h1");
3let t = document.createTextNode("Hello World");
4myNewHOne.appendChild(t);  // This adds the text content to the <h1>  
5divElem.appendChild(myNewHOne); // This adds the <h1> into the "myFirstDiv" element

Now, let's say that we have a <ul> with 2 elements, but we want to insert a new <li> at the beginning of that list.

We can use the function insertBefore for that case – like this:

1...
2let newItem = document.createElement("li");
3let textNode = document.createTextNode("Water");
4newItem.appendChild(textNode);
5let list = document.getElementById("myList");
6list.insertBefore(newItem, list.childNodes[0]);  // adding the newItem before the FIRST child of the list

innerHTML

As you already know, each element in the HTML document can have some sort of HTML content. It does not matter if it is a <p>, <div>, <a> or any other HTML element; it can have its own innerHTML combined with more HTML content.

what is dom

The .innerHTML property gives you the ability to retrieve or set the content of whatever element you have in your JavaScript. For example:

1document.getElementsByTagName("div")[0].innerHTML = "abc";
2// innerHTML can be used to insert plain text content or HTML, this creates a list inside a <div> element

It can also be used to add entire new elements into your HTML without being so verbose as the createElement examples above. Let's rework the `"Hello World" to see the difference:

1    document.querySelector("myFirstDiv").innerHTML += `<h1>Hello World</h1>`

Dramatic difference!!! We achieved the same result with one line of code that previously required 5 lines.

By using template literals (``) we can even break down the HTML elements onto separate rows, so it is easier to read them, and also we can use variable values, instead of hard-coded ones:

1    let greeting = "Hello World"
2    document.querySelector("myFirstDiv").innerHTML += `
3    <div>
4        <h1>
5            ${greeting}
6        </h1>
7    </div>
8    `

☝️ Get into the habit of using this approach as it is much more concise and straightforward which also makes your code easier to read.

You can find another 2 properties on the internet: nodeValue and textContent, but they are not really universally used and are more limited in functionality.

Removing Elements from the Document

The most straightforward method to remove an element from the DOM is the remove() method.

For example, if we want to remove a <p> element with id="firstP", we can do the following:

1// Removing all children from an element  
2let element = document.querySelector("p#firstP");
3element.remove();

Changing Attributes

To change any attribute of any object in the DOM, we need to use the .attribute property just like this:

1// Changing attributes  
2let element = document.querySelector("#someId");
3element.someAttribute = "newValue";

For example, to change the value of a button element, you would need to code this:

1// Changing value attribute  
2let btn = document.querySelector("#myButton");
3btn.value = "newButton";

Changing the Styles

You can also change any CSS rule or property applied to the HTML elements by using the .style attribute, like this:

1// Changing styles  
2let element = document.querySelector("#myElementId");
3element.style.color = "red";
4element.style.backgroundColor = "blue";

☝️ Notice how css properties which consist of two words, like background-color, become one word in camel case in the DOM - backgroundColor.

Changing Classes

Many times as you dynamically change the content of your application, you would want to change the classes of some elements, because classes allow us to give our elements different styling and functionality.

Imagine that you are given a bootstrap button:

1<button id="myButton" class="btn btn-warning">Submit</button>

You would like to change its background color from yellow to red at some point of your application's runtime. You can do that by simply exchanging its bootstrap classes for background color:

1// Changing classes  
2let btn = document.querySelector("#myButton");
3btn.classList.remove("btn-warning");
4btn.classList.add("btn-danger");

You can do the same thing with your own custom classes.

As you can see, the classList property gives you access to the classes of a given element, and you can add or remove classes.

With this knowledge, now research and make sure to understand the method classList.toggle("newClass").

Further reading

For more information about accessing the DOM, see: https://developer.mozilla.org/en-US/docs/Web/API/Document