Understanding DOM Rendering in 10 points

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DOM Rendering

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The DOM (Document Object Model) is a key concept for understanding how web pages work and how content is rendered.

This article explains DOM Rendering and its importance in the creation of high-performance websites.

1. What is the DOM?

Le DOM is a programming interface (API) that represents the structure of a web page in the form of a tree made up of nodes, corresponding to the elements HTML of the page. Each node is an object with properties and methods for manipulating these elements and their attributes.

Defining the content of a web page

With the DOMThis allows developers to create dynamic interactions between page elements and user actions. Developers can thus create dynamic interactions between page elements and user actions, such as adding or deleting elements, modifying attributes and styles, or managing events such as clicks or mouse movements.

2. How does DOM Rendering work?

The process of DOM Rendering takes place in several stages:

1. Loading the HTML document: The browser reads and analyses the HTML source code of the web page and creates a representation in memory in the form of a DOM tree.
2. Building the rendering tree: The browser creates another tree called the "rendering tree", which is a more concrete representation of the document, including the style elements (CSS) and object dimensions.
3. Painting: The final image is drawn on the screen following the rendering tree, layer by layer.

Importance of DOM Rendering optimisation

The process of DOM Rendering can be optimised to improve the responsiveness and performance of a web page. Optimisation techniques frequently used include :

• Reduce the number of DOM elements and the depth of the tree.
• Use efficient DOM update methods and avoid unnecessary modifications.
• Avoid or minimise forced manipulations of the render tree, also known as "reflow".

3. The different types of node in the DOM

There are several types of nodes in the DOMincluding :

• Element nodes, which correspond to HTML tags.
• Text nodes, which contain the textual content of the elements.
• Attribute nodes, representing the attributes of elements (for example, the "src" attribute of an image).

4. Relationships between nodes: parent, child and brother

The nodes of the DOM are linked together by parent-child or sibling relationships, making it easy to navigate the DOM tree:

• A parent node is an element that contains other elements.
• A child node is an element contained within another element.
• Two nodes are brothers if they are both children of the same parent node.

5. DOM manipulation with JavaScript

The language JavaScript offers a wide range of methods and properties for interacting with the DOM and dynamically modify the content and presentation of web pages. Common operations include :

• Select items.
• Create, add, delete or replace elements and attributes.
• Modify the content and styles of elements.
• Manage events and their propagation.

6. Selecting elements with the DOM

Several methods of selecting elements are available in JavaScript for easy access to objects DOM :

• getElementById(): selects an element by its identifier ("id" attribute).
• querySelectorAll() and querySelector(): select elements using CSS selector syntax.
• The parentNode, firstChild, lastChild, previousSibling and nextSibling properties enable navigation in the DOM tree from a given node.

7. Creating and adding elements to the DOM

To create and insert a new element in the DOMyou can use the createElement(), createTextNode() and appendChild() methods:

1. Creation of a new DOM object corresponding to the desired element using createElement().
2. Define the attributes and text content of the element with createTextNode().
3. The element is added to the DOM tree at the desired position using appendChild().

8. Deleting or replacing DOM elements

Elements can also be removed and replaced using the removeChild() and replaceChild() methods:

• removeChild(): removes an element from the DOM tree, thereby freeing the associated memory.
• replaceChild(): replaces an existing element with another previously created element.

9. Modifying styles and classes with the DOM

Objects DOM are used to access the style properties (_style_) and the list of classes (_classList_) of elements, in order to dynamically modify their presentation:

• The style of an element can be modified by defining a new value for a given property or by adding an entire style rule.
• The list of classes provides methods for adding, deleting, swapping or checking the presence of a CSS class on an element.

10. Managing DOM events and their propagation

The interactions between users and web pages are often based on DOM eventsevents, such as clicks, mouse movements, keyboard strokes, etc. To manage these events, you can :

• Attach event listeners to the elements concerned, which will trigger a specific function when an event occurs.
• Control the propagation of events, for example by stopping the transmission of an event to its ancestors (_stopPropagation()_ method).

As a result, the DOM Rendering is an essential concept in the creation of high-performance, interactive websites. Understanding and mastering it is essential for any web developer wishing to create dynamic web pages tailored to users' needs.