The Document Object Model

Javascript running in the web browser has access to the currently displayed web page via an interface called the Document Object Model or DOM. The DOM interface is object based as it's name suggests and allows a script to locate any part of the page, read the contents and update it either by changing text or adding or removing elements in the page.

The DOM is the way that the browser represents the page being viewed. It is an interpretation of the HTML structure as a hierarchical tree of objects. The main objects in the tree represent the elements in the HTML page, other nodes represent attributes and the textual content of the elements. Here is a simple example taken from the W3C (What is the Document Object Model) corresponding to the following HTML fragment:

<table>
  <tbody> 
    <tr> 
      <td>Shady Grove</td>
      <td>Aeolian</td> 
    </tr> 
    <tr>
      <td>Over the River, Charlie</td>        
      <td>Dorian</td> 
    </tr> 
  </tbody>
</table>

DOM Example

Note that the element nodes are arranged in a tree structure, the table element contains one tbody and that contains two tr nodes. We talk about these as the children of the parent nodes. table has one child and tbody has two. The rounded nodes are those containing the text inside the elements; these are the leaves of the tree.

In Javascript, we have a standard interface to the DOM that is implemented by all browsers and can be used to both read the contents of the current page and update the page with new or modified content.

In a Javascript program running in the browser, the current document object can be accessed by the global variable document which is an instance of the class Document. This is the root of the tree and all operations on the DOM are carried out via method calls on document.

This chapter will not provide an exhaustive tutorial on the DOM. You should look at other resources for that, for example the MDN Tutorials on Manipulating Documents or the W3Schools HTML DOM tutorial Instead, this chapter will point out some of the main concepts relating to our use of the DOM in building web applications.

Finding parts of the page

There are two main ways to locate elements in the DOM from Javascript: you can navigate from the root document step by step via child links to find the element you want or you can use methods to search the tree for the element. This second option is much more useful and more commonly used.

To find the children of the document node we can use the property document.children which is a list of child nodes. In the case of the document node there should be one child which is the root of the document - the <html></html> element. This element can have more children - typically it will have head and body elements below it. The following code will show these children:

var root = document.children[0]
var nodes = root.children

for(var i=0; i<nodes.length; i++) {
    console.log(nodes[i])
}

Note that you can try these examples out in your web browser. Open the developer tools and select the Javascript Console. You can enter these commands at the prompt to explore the DOM representation of the page that is loaded in your browser. If you print an element with console.log you will get a representation of the content of the element that you can explore as a DOM tree.

Screenshot of the Javascript console in Chrome

A more common operation is to search for the element you are interested in using either the element name, class or identifier (id). The document object has methods for each of these options illustrated in the following example:

// get all of the h1 elements in the document
var headers = document.getElementsByTagName("h1")
// get all of the elements with class='important'
var important = document.getElementsByClassName('important')
// get the element with id='content'
var content = document.getElementById('content')

These methods return either an array-like object (the first two getElementsBy methods) or a single node (the last one getElementBy). Each of these nodes has a similar set of methods to the document object (they are instances of Node rather than Document but provide a similar interface). So, you can look at their children or even search within the element for other elements. In this example we find an element with a particular id and then search within that for all of the table data (td) elements:


var table = document.getElementById('resulttable')
var tdlist = table.getElementByTagName('td')
for (var i=0; i<tdlist.length; i++) {
    console.log(tdlist[i].innerHTML)
}

This example also shows another part of the node interface. I can find the content of a node as an HTML string via the innerHTML property. This is a very useful property because it also allows us to update the content of the element to modify the page as we'll see in the next section. Another similar property is innerText which is the content of the element as text, with any HTML tags removed.

Modifying the Page

As mentioned, the innerHTML property of a node in the DOM corresponds to the HTML content of an element as a string. This is most useful when we want to change the content of the element from our script. This is just one way that we can modify the current page from Javascript. A common practice in Javascript driven web applications is to provide a placeholder in a page with a given id attribute that will be filled with different content from the script as it executes. So I might have a target div element like this:

<div id="content"></div>

and target this from my Javascript:

var contentdiv = document.getElementById('content')
var newcontent = "<p>Demonstrating insertion of content.</p>"

contentdiv.innerHTML = newcontent

There are other methods of modifying the content of an element, for example, the following code will create a new paragraph tag and add some text content to it, then insert it inside the content div selected above.

var newp = document.createElement('p')
newp.innerText = "Another example of inserting content"
contentdiv.appendChild(newp)

These examples illustrate the way in which Javascript can update the current page being shown in the browser. Any changes that are made to the document are reflected live in the page. This is the basis of our ability to write Javascript applications with rich user interfaces running inside the web browser.

Exercise: Fake News

This exercise is about modifying the content of a page in your web browser using Javascript. As we have said, Javascript has access to the current page via the Document Object Model or DOM. In your browser, the variable document represents the current document and your code can use it to read and write to the current page. In this exercise we will use the developer tools Javascript console to write Javascript to access parts of the main page of a newspaper site and, just for fun, modify their contents. The instructions here relate to Chrome but some variation will work in any browser.

  • Go to the main page of a news website, eg. SMH, Guardian, Telegraph, Buzzfeed or whatever (it should have headlines on the main page)
  • Right click on one of the headlines and choose "Inspect"

This should show you the HTML of the page with the element corresponding to your headline highlighted. Take note of the kind of element it is (eg. <h3>) and any class attributes it has. You will use these to select this element in your Javascript code.

Switch to the Console tab in developer tools The console allows you to run Javascript code in the context of the current page, so the document variable will represent the page you are viewing. Write Javascript code to select all of the headline elements, so if you saw that they were all <h3> elements you might write:

headlines = document.getElementsByTagName("h3")

Take the first of these and find the HTML content of the heading:

headlines[0].innerHTML

Now modify this to add your own message to the page

headlines[0].innerHTML = "COMP249 Found to be Best Unit in the Universe!"

Check the page to see your updated headline.

Ok, so we can fake the news, that's good but we can go further.

  1. Write a for loop that changes the content of all of the headlines to your chosen message
  2. Write a function that takes your new message as an argument and modifies the headlines when it is called

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