JavaScript DOM | Learn Computer Academy

What is the DOM?

DOM stands for Document Object Model. It’s a tree-like structure that represents the contents of an HTML document.

When a webpage loads, the browser turns your HTML into a DOM tree, where:

Each HTML element becomes an object
JavaScript can access, update, add, or delete these elements

📌 In short: JavaScript uses the DOM to interact with your web page dynamically.

Visualizing the DOM Tree

<html>
  <body>
    <h1>Hello</h1>
    <p>This is a paragraph.</p>
  </body>
</html>

Document
└── html
    └── body
        ├── h1
        └── p

Accessing Elements in the DOM

Method	Description
`getElementById()`	Finds element by ID
`getElementsByClassName()`	Finds elements by class name
`getElementsByTagName()`	Finds elements by tag
`querySelector()`	Finds first matching element (CSS)
`querySelectorAll()`	Finds all matching elements (CSS)

<p id="demo" class="text">Hello!</p>

document.getElementById("demo"); // By ID
document.getElementsByClassName("text");    // By class
document.querySelector("p");                // First <p>
document.querySelectorAll("p");             // All <p>

Changing Content with JavaScript

innerHTML

Replaces the HTML content inside an element.

document.getElementById("demo").innerHTML = "Updated!";

textContent

Changes only the text (ignores HTML tags).

document.getElementById("demo").textContent = "Plain Text";

Changing Style

const box = document.getElementById("box");
box.style.backgroundColor = "blue";
box.style.fontSize = "20px";

Creating and Appending Elements

const newP = document.createElement("p");
newP.textContent = "This is a new paragraph.";
                                  
document.body.appendChild(newP);

Removing Elements

const item = document.getElementById("removeMe");
item.remove();

Or using parent element:

item.parentNode.removeChild(item);

DOM Events Recap

You can combine DOM selection with events:

document.getElementById("btn").addEventListener("click", function () {
  document.getElementById("demo").textContent = "Button clicked!";
});

Traversing the DOM

Property	Description
`parentNode`	Access parent element
`children`	Access all child elements
`firstElementChild`	First child element
`lastElementChild`	Last child element
`nextElementSibling`	Next element at the same level
`previousElementSibling`	Previous element at same level

const list = document.getElementById("myList");
console.log(list.children); // HTMLCollection of li elements

Attributes and Classes

Getting & Setting Attributes

const link = document.getElementById("myLink");
console.log(link.getAttribute("href")); // Get
                                  
link.setAttribute("href", "https://example.com"); // Set

Working with Classes

const box = document.querySelector(".box");

box.classList.add("active");    // Add class
box.classList.remove("box");    // Remove class
box.classList.toggle("hidden"); // Toggle class

🧪 Practice Exercise:

Task:

Change the text of a heading using innerHTML.
Create a new <li> item and add it to an unordered list.
Toggle a class on button click.
Remove an element after a few seconds using setTimeout().
Change the src attribute of an image dynamically.