{ Introduction to the DOM. }


By the end of this chapter, you should be able to:

  • Explain in your own words what the DOM is and how it is created
  • Use the document object to manipulate the DOM
  • Iterate over DOM elements and add attributes


Before you continue onto the next section, make sure you have a solid understanding of HTML. We will be using JavaScript to manipulate HTML so if you do not know what HTML elements and attributes are, this section is going to be challenging. You can work through the first couple sections of Code Academy HTML + CSS to get a better understanding, or check out an excellent tutorial by Shay Howe here

What is the DOM?

From MDN:

The Document Object Model (DOM) is a programming interface for HTML, XML and SVG documents. It provides a structured representation of the document as a tree. The DOM defines methods that allow access to the tree, so that they can change the document structure, style and content. The DOM provides a representation of the document as a structured group of nodes and objects, possessing letious properties and methods. Nodes can also have event handlers attached to them, and once an event is triggered, the event handlers get executed. Essentially, it connects web pages to scripts or programming languages.

To access the DOM, we make use of the document object. This object has properties and functions that we use to access our HTML elements which we can manipulate with JavaScript.

Working with the DOM is one of the more fun parts about using JavaScript because we can add behavior to our web pages. With JavaScript, we can change how a page looks and functions based on events the user makes (clicking, hovering, submitting a form, typing a certain key). As you walk through the next couple sections, think about what you might be able to build with this new knowledge.

How to access elements in the DOM

Let's get started with this sample HTML. Copy and paste this into a file called index.html and add an external script tag, or use a <script></script> right before the body closes. For simplicity, we'll take the latter approach right now.

<!DOCTYPE html>
<html lang="en">
    <meta charset="UTF-8">
    <!-- make sure for now that your script tag is in the head -->
<body id= "container">
    <div class="hello">
        Hello World
    <div class="hello">
        Hello Everyone!
    <a href="#">This link goes nowhere!</a>
    <button>Click me!</button>
        // you can put your JavaScript in here, or include an external file.

The easiest way to select elements is by their id using the getElementById function on the document object (document.getElementById). This returns a SINGLE element (because ids must be unique!).

let container = document.getElementById("container");

We can also use a function called querySelector, which selects a SINGLE element using CSS selectors. If multiple elements match the query you pass in to querySelector, the function will simply return the first matching element that it finds.

let container = document.querySelector("#container");

Notice that when we select by id using querySelector, we pass in the string #container, not container. Remember: querySelector always expects a CSS selector. In contrast, when we use getElementById, we just pass in the string container (no hashtag)! Since getElementById expects to find an element by id, in this case the hashtag isn't necessary.

To select multiple elements, we can use getElementsByTagName or getElementsByClassName, or we can use querySelectorAll and pass in a CSS selector. These will return what appear to be arrays (they are not exactly arrays, but for right now, that is not a problem).

let divs = document.getElementsByTagName("div");
let divs = document.querySelectorAll("div");

Here is another example using getElementsByClassName and the same thing with querySelectorAll.

let divsWithClassOfHello = document.getElementsByClassName("hello");
let divsWithClassOfHello = document.querySelectorAll(".hello");

As you can see, when you pass in a class name using getElementsByClassName, you don't need to start the string with a dot. The function expects to receive a class name - it's in the name of the function! On the other hand, querySelectorAll takes in any valid CSS query, which is why you need to pass in .hello if you want it to find all elements with a class of "hello."

When you're ready, move on to DOM Manipulation


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