DOM (Document Object Model) is a structured tree model that the browser converts HTML documents into, which is used to dynamically operate page content of the program. It treats each element, attribute, and text as an object with hierarchical relationships, forming a family tree-like structure, such as as the root node and
and as its child nodes. HTML is static text, while DOM is an operable interface generated by the browser after parsing HTML, allowing JavaScript to access and modify page content. Common operations include: 1. document.getElementById() gets the element; 2. document.querySelector() selects the first matching element; 3. element.innerHTML modify content; 4. element.addEventListener() binds the event. Notes include: ID must be unique; ensure that the DOM is loaded before executing the script; avoid frequent operation of the DOM to improve performance.
After the web page is loaded, the browser will convert HTML into a structured model to facilitate program operation and modify page content. This model is DOM (Document Object Model), which is essentially a tree-like representation of HTML documents.
What is DOM?
DOM treats every element, attribute, and even text in an HTML page as an object. There is a hierarchical relationship between these objects, forming a "tree". For example, is the root node, and its children are and , and there may be multiple child elements in , such as <div> , <code><p></p> , etc.
You can think of it as a genealogy: parent node, child node, brother node, and this structure is clear and intuitive. This way the browser understands the page structure and allows JavaScript to dynamically access and modify page content.
The relationship between DOM and HTML
HTML is a static text file, and it displays whatever you write. But once the browser parses this HTML, the corresponding DOM tree will be generated. This tree can be operated by JavaScript.
To give a simple example:
<p>Hello</p>
In HTML, it's just a tag. But in the DOM, it is an object that has properties such as tagName and textContent , and can also bind events, such as executing a piece of code when clicking.
In other words, HTML is the source data, and DOM is the browser's understanding and operation interface for it.
How to operate DOM with JavaScript?
JavaScript provides many ways to access and modify the DOM. Common ones include:
-
document.getElementById(): Get element based on ID -
document.querySelector(): Use CSS selector to get the first matching element -
element.innerHTML: Modify the HTML content inside the element -
element.addEventListener(): Add an event listener to the element
For example, if you want to change the text content of a paragraph, you can write it like this:
let para = document.getElementById("myParagraph");
para.textContent = "The content has changed!"; This line of code finds the element with ID myParagraph and then modify its text content. This is a common way for DOM to update pages dynamically.
Common precautions
- ID must be unique : if there is a duplicate ID on the page,
getElementByIdmay not work as expected. - DOM Loading Order : If your JS runs before an HTML element, the corresponding element may not be found. It is recommended to place the script in the
DOMContentLoadedevent, or at the end of. - Performance issues : Frequent operation of DOM will affect the performance of the page. For example, continuously modifying the node content in a large number of loops. It is best to process it in memory first and then insert it once.
DOM is one of the foundations of front-end development. Understanding how it cooperates with HTML can give you better control over page behavior. Basically all that is, it’s not difficult but it’s very critical.
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