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An Introduction to the Document Object Model (DOM)

Published by Nathaniel Stickman
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The Document Object Model (DOM) is a language-agnostic interface that provides access to an HTML document’s structure and content. JavaScript is often the language used to access the DOM in order to generate dynamic and interactive web pages.

This guide provides an overview of the Document Object Model and shows you how to interact with it using a series of JavaScript examples.

Before You Begin

The examples in this guide use a web browser’s developer tools to view the DOM and interact with a JavaScript console. To get the most out of the information in this guide, follow along in your own Chrome or Firefox browser.

  • On Chrome, refer to Google’s Open Chrome DevTools documentation to learn how to access their developer tools.

  • On Firefox, refer to Mozilla’s Open the Inspector documentation to learn how to access their developer tools.

What Is the Document Object Model?

The Document Object Model (DOM) is an interface that provides access to an HTML document’s structure and content. The DOM represents the elements and content of an HTML document as nodes and objects. The DOM representation can then be accessed and modified by JavaScript and other scripting languages.

Essentially, the DOM is what allows web pages to become dynamic. Languages like JavaScript work with the nodes that make up the DOM to dynamically and interactively change a web page’s presentation.

There are many ways of displaying the DOM. One of the most widely used ways is the HTML format. When you open your browser’s Inspect dashboard, you can view the HTML representation of a web page. The example below shows the HTML markup for a simple web page.

<!DOCTYPE html>
<html>
  <head>
    <title>Example Page</title>
  </head>
  <body>
    <p>Example page content.</p>
  </body>
</html>

How the DOM Differs from HTML Source Code

The DOM itself is not equivalent to a web page’s HTML source code. Instead, the DOM is a representation of how a web page is displayed in the moment that it is accessed.

To illustrate how the DOM and HTML source code can differ, the example below displays an HTML source file. The HTML file includes a JavaScript function that adds additional HTML elements to the page once the page loads.

File: example-page.html
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<!DOCTYPE html>
<html>
  <head>
    <title>Example Page</title>
    <script>
        function addExampleList() {
            const exampleList = document.createElement("ul");

            const exampleListItem1 = document.createElement("li");
            const exampleListItem1Text = document.createTextNode("First item");
            exampleListItem1.appendChild(exampleListItem1Text);

            const exampleListItem2 = document.createElement("li");
            const exampleListItem2Text = document.createTextNode("second item");
            exampleListItem2.appendChild(exampleListItem2Text);

            exampleList.appendChild(exampleListItem1);
            exampleList.appendChild(exampleListItem2);

            document.body.appendChild(exampleList);
        }
    </script>
  </head>
  <body onload="addExampleList();">
    <p>Example page content.</p>
  </body>
</html>

Once the HTML page is loaded and the JavaScript runs, the DOM representation of the HTML source above resembles the code displayed below. The JavaScript has been left out to make the resulting HTML easier to read. The HTML now includes an unordered list (<ul>...</ul>) with two list items (<li>...</li>).

<!DOCTYPE html>
<html>
    <head>
        <title>Example Page</title>
        <script>[...]</script>
    </head>
    <body onload="addExampleList();">
        <p>Example page content.</p>
        <ul>
            <li>First item</li>
            <li>Second item</li>
        </ul>
    </body>
</html>

Since the DOM is concerned with displaying the current state of an HTML page, it now displays the new HTML elements that were added to the page by the JavaScript. The DOM always reflects any additions, subtractions, or other modifications that happen to a web page. This characteristic is what enables the DOM to make web pages dynamic.

The Document Object Model and JavaScript

Most often, JavaScript is how web developers interact with the DOM. JavaScript is able to access the DOM with the document object and the nodes nested under it.

The next sections explain what the document object is and the parts that make it up.

Document Object

To work with the DOM, client-side JavaScript provides the document object. This object includes properties and methods to access and modify the DOM.

The previous section included some examples of the document object in action. Below are two additional commands that show more of the document object’s features.

  1. The document object’s properties provide information about the HTML document or access to its nested nodes. They also allow you to modify characteristics of the DOM as shown in the example below:

    document.body.style.backgroundColor = "blue";

    The example JavaScript accesses the document object’s backgroundColor property and sets its value to "blue". The web page it modifies should now have a blue background. The DOM representation of the change looks as follows:

    <!DOCTYPE html>
<html>
    <head>
        <title>Example Page</title>
        <script>[...]</script>
    </head>
    <body onload="addExampleList();" style="background-color: blue;">
        <p>Example page content.</p>
        <ul>
            <li>First item</li>
            <li>second item</li>
        </ul>
    </body>
</html>

    The color blue is assigned to the <body> element using the style attribute.

  2. The document object has several methods that do everything from provide access to specific nodes to add new nodes to the DOM. In the example below, the getElementsByTagName() method grabs every HTML element with the tag name, <li>. The JavaScript loops through those elements, and then outputs each elements textContent attributes.

    for (item of document.getElementsByTagName("li")) {
    console.log(item.textContent);
}

    Using the for loop above, the JavaScript console should display the following output:

    First item
Second item

Nodes and Elements

The document object contains numerous other objects that all make up the DOM. These objects are called nodes. Nodes include everything from HTML elements, to attributes, to text.

You are likely to work most frequently with element nodes. DOM element nodes correspond to a web page’s HTML elements. They allow you to access and manipulate the building blocks of a web page.

The script used in the How the DOM Differs from HTML Source section added a <ul> element and <li> elements to the page. This added the following two kinds of nodes to the page:

  • Element nodes, which were created using the document.createElement method.
  • Text nodes, created with the document.createTextNode method.

Each part of the document object is actually a node of some kind or other. Additionally, each node inherits common properties, like the appendChild method, which lets elements add text nodes.

The document object does more than just let you extend the DOM. For instance, you can also use it to navigate the DOM and make precise modifications to it. The script below demonstrates how these modifications can be made to the DOM. Access the example-page.html page in your browser. Then, open your browser’s JavaScript console, and enter in the following JavaScript:

const listItems = document.getElementsByTagName("li");

for (item of listItems) {
    const newTextNode = document.createTextNode(item.textContent.replace("item", "thing"));

    item.innerHTML = "";
    item.appendChild(newTextNode);
}

As a result, the DOM is updated and the text, item, contained within the <li> tags is updated to thing.

<!DOCTYPE html>
<html>
    <head>
        <title>Example Page</title>
        <script>[...]</script>
      </head>
      <body onload="addExampleList();">
        <p>Example page content.</p>
        <ul>
            <li>First thing</li>
            <li>Second thing</li>
        </ul>
    </body>
</html>

See our guide Traversing the Document Object Model with JavaScript, to learn about other built-in document object methods.

Conclusion

The DOM provides an interface to an HTML web page. This enables you to manipulate the structure and content of a web page using scripting languages, like JavaScript. This guide introduced you to the DOM and demonstrated how JavaScript is used to add, modify, and remove HTML elements from a web page.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

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