DOM Manipulation

Definition of DOM (Document Object Model)

The DOM (Document Object Model) is a programming interface for HTML and XML documents. It represents the structure of a document as a tree, where each HTML element (text, images, links, etc.) is a manipulable node. The DOM thus allows programming languages, like JavaScript, to access and dynamically modify the structure, style, and content of a web page.

DOM Structure

Root node: the <html> element is the entry point of the DOM tree.
Child nodes: each element (tag) in HTML can have children (like <body>, <div>, etc.), forming a hierarchical structure.
Text nodes: the content of tags (like text in <p>) is also considered as a distinct node.

Accessing DOM elements with JavaScript

JavaScript allows access to DOM elements through methods that target tags according to their identifier, class, tag, or CSS selector. Here are the most common methods:

document.getElementById("id"): selects a unique element by its identifier.
document.getElementsByClassName("class"): selects all elements having a specific class.
document.getElementsByTagName("tag"): selects all elements of a certain tag type.
document.querySelector("selector"): selects the first element matching a CSS selector.
document.querySelectorAll("selector"): selects all elements matching a CSS selector.

Example

// Selection of an element by ID
let title = document.getElementById("title");
console.log(title.textContent);

Modifying content and attributes of elements

Once an element is selected, JavaScript allows modifying its content, attributes, and style.

Change text: .textContent or .innerHTML
Modify an attribute: .setAttribute("attribute", "value")
Change style: .style.property = "value"

Example

// Change the text of an element
title.textContent = "New Title";

// Modify an attribute
title.setAttribute("class", "new-class");

// Change style
title.style.color = "blue";

Adding and removing elements

JavaScript also allows dynamically adding or removing elements in the DOM.

Create a new element: document.createElement("tag")
Add the element: parentElement.appendChild(newElement)
Remove an element: parentElement.removeChild(childElement)

Example

// Create a new <p> element and add it to the <div> with ID "container"
let newParagraph = document.createElement("p");
newParagraph.textContent = "This is a new paragraph.";
document.getElementById("container").appendChild(newParagraph);

// Remove an existing element
document.getElementById("container").removeChild(newParagraph);

Manipulating events in the DOM

Events allow triggering actions in response to user interactions, such as a click or mouse hover.

Adding an Event Listener

Recommended method: .addEventListener("event", function)

Example

// Add an event listener to the button to display an alert on click
let button = document.getElementById("myButton");
button.addEventListener("click", function() {
    alert("Button clicked!");
});

Application examples

Display a message when clicking a button.
Change the color of an element when hovering the mouse over it.
Add a task to a list by clicking a button.

Conclusion

DOM manipulation with JavaScript is essential for creating interactive web pages. By accessing and modifying DOM elements, it is possible to dynamically transform the content, structure and style of web pages, thus offering a better user experience.