javascript dom
Post on 21-May-2017
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Javascript DOM
Peter Atkinson
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Objectives
• Understand the nature and structure of the DOM
• Add and remove content from the page • Access and change element attributes –
including source and class• Insert markup into a page using
innerHTML• Change style attribute using Javascript
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What is the DOM?
• Browser Wars 1997• W3C standardised DOM 1998• End of ‘browser sniffing’ • Document Object Model – an API that can
be used by any language. A knowledge of the DOM will help with PHP or XML
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Nodes Organise the Page<html><head> <title>My page</title></head><body> <p>This is text on my
page</p></body></html>
html
head body
title
text
“my page”
p
text
“This is text on my page”
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What is a Node?
• Element Node – contains an HTML tag• Text Node – contains text• Text Nodes are contained in Element
Nodes
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Adding Some Text To A Page
There are five steps:1. Create a new Element2. Create new Text3. Append the new Text to the new Element4. Find an existing Element5. Append the new Element to the existing
Element
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1. Create New Element Node
Let us, say create a new <p> tag (element) so that we can attach some text to it
For convenience, we can put the new object into a variable
var newNode;newNode = document.createElement(“p”)
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2. Create a Text Node
Next, create a text nodeAgain, for convenience, we can put the new
text node into a variablevar newText;newText = document.createTextNode(“Some text.”)
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3. Attach the New Text Node to the New Element
To put the text into the page, we have to attach the text node to the new HTML element:
newNode.appendChild(newText);
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4.Find an Existing Element
The new element with our text node attached to it is still floating around in a Javascript world. We need to find an existing element so that we can attach it
For convenience, we shall put this existing element into a variable
var docElement;docElement = document.getElementById(“thisLocation”);
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5. Append the New Element to the Existing Element
To insert our text into the page, we now have to append the new element to the existing element
docElement.appendChild(newNode);
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Putting the 5 Steps TogetherHands On: Try out this code
<head><script language="javascript" type="text/javascript">var myText;myText = "This is new text to be added to the page dynamically.";function addText(location) {
var newNode;var newText;var docElement;newNode = document.createElement("p");newText = document.createTextNode(myText);newNode.appendChild(newText);docElement = document.getElementById(location);docElement.appendChild(newNode);
}</script></head><body><p><a href="#" onclick="addText('thisLocation');">Click to add new text to the page</a></p><p id="thisLocation">New text will appear below here</p><p>Some further text in the page</p></body>
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Remove a Node• To remove a node, we use the element method
removeChild(name of node to be removed)• For example:function remText(location) {
var docElement; docElement = document.getElementById(location); docElement.removeChild(docElement.lastChild);
}Hands OnModify your HTML page so that the user can click on some
text to remove the text that was inserted
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getElementsByTagName()
• getElementById() allows you to work with elements by their individual id but often you will want to work with a group of elements
• getElementsByTagName() allows you to work with groups of elements. This method returns an array
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Using getElementsByTagName()
• Hands On• Open the file JavascriptDOM1.html• Insert this code at the bottom of the document
body:<script language="javascript" type="text/javascript">theseElements = new Array;theseElements = document.getElementsByTagName("li");alert(theseElements.length);</script>
• Now try substituting the tag name li with the wild card *. Try the code in both IE and Firefox.
• Are you surprised by the number of nodes?
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Stepping Through an Array of Nodes
• We can step through the array of nodes and check what kind of node it is:for (i = 0; i < theseItems.length; i++) { alert(typeof theseItems[i]);}
Hands OnAdd this code to JavascriptDOM1.htmlExecute the code
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Where on the Node Tree?
• childNodes– nodeList = node.childNodes
• firstChild– reference = node.firstChild
• lastChild• nextSibling• parentNode• previousSibling
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Attribute Nodes
• We can get at the attributes of an element through attribute nodes
• Attribute nodes, like text nodes are always contained in element nodes
• We shall look at methods:– getAttribute()– setAttribute()
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Getting Attribute Nodes• Hands On• Open the file JavascriptDOM2.html• Add this code to alert the attribute of an element:
function dispAttribs() {var messg;attribs = new Array;attribs = document.getElementsByTagName("p");for (i = 0; i < attribs.length; i++) {
messg = attribs[i].getAttribute("className");alert(messg);}
}• Add this to the bottom of the body:
<p onclick="dispAttribs();">Click here to see class attributes</p>• Try this in Firefox• Point to consider: why is this attribute called ‘className’?
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Setting Attribute Nodes• Hands On• Open the file JavascriptDOM2.html• Add this code to change the attribute of an element:
function chngAttribs() {var messg;
attribs = new Array;attribs = document.getElementsByTagName("p");for (i = 0; i < attribs.length; i++) {
attribs[i].setAttribute("className","jazz");}
}• Add this to the bottom of the body:
<p onclick="chngAttribs();">Click here to change class attributes</p>
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User inserts and removes text
• Hands On• Use file JavascriptDOM3.html• Place code in this page so that:
– When the user mouseovers on an image, the relevant text appears
– When the user mouseouts on an image, the text disappears
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Accessing Images• Hands On• Open JavascriptDOM4.html• Examine the layout of the page• We are going to modify the behaviour of the
page so that instead of the images displaying in a new window, they display on the same page
• Write a function that will alter the source of the placeholder image on the page to another image
• Call this function from the onclick event of each link
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Suggested Solution
• Hint:• Get the value of the attribute href from the
link• Find the placeholder node• Set the src attribute of the placeholder to
be the same value as the href attribute of the link
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Code
function showPic(whichpic) {var source = whichpic.getAttribute("href");var placeholder = document.getElementById("placeholder");placeholder.setAttribute("src",source);
}In each link, we need an onclick event: onclick="showPic(this);return false;"
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Web Page Architecture
• Graceful Degradation = your site is navigable by users who do not have Javascript
• Progressive Enhancement = page built in layers:
HTML for Markup
CSS for Presentation
Javascript for Functionality
Gra
cefu
l Deg
rada
tion
Pro
gres
sive
Enh
ance
men
t
The page should be constructed using 3 different files – one for each layer
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Graceful Degradation
• Does this degrade gracefully?<a href=“#” onclick=“popUp(‘http://www.example.com’); return
false;”)>Link to Example</a>
This does degrade gracefully:<a href=“http://www.example.com”
onclick=“popUp(‘http://www.example.com’); return false;”)>Link to Example</a>
But it is a little clumsy. There is a shortcut:<a href=“http://www.example.com” onclick=“popUp(this.href);
return false;”)>Link to Example</a>
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Progressive Enhancement
• Graceful Degradation follows from Progressive Enhancement
• We need to separate the Javascript from the markup by removing the event handlers from the HTML completely
• We can attach events to HTML tags in the Javascript using the attributes class and id
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Unobtrusive Javascript• Examine this code:
window.onload = prepareLinks;function prepareLinks() {
var links = document.getElementsByTagName(‘a’);for (var i=0; i<links.length; i++) {
if (links[i].className == “popup”) {links[i].onclick = function() {
popUp(this.getAttribute(“href”));return false;}
}}
}
Attaches code to the onclick event of tags which have been identified by their class name
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Using Unobtrusive Javascript• Hands On• Open JavascriptDOM4.html• Remove the onclick event handlers• In each a tag put the attribute class=“popup”• Enter the code from the example given so that it
is unobtrusively called from the onclick event• Also enter this code to open a popup
window:function popUp(winURL) {
window.open(winURL,”popup”,”width=320,height=480”);}
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Backwards Compatibility
• Although most browsers fully support the DOM, some do not support it completely.
• Browser sniffing is too convoluted, so best to check for specific features
• Put this line of code at the beginning of a function if (!document.getElementsByTagName) return false;
• So, if the browser does not support this method the function will stop
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Tidying Up• Hands On• To follow through the principle of completely separating
the three layers, we need to put all our Javascript in a separate file
• Open JavascriptDOM4.html• Put the Javascript code into a new file called
javascriptdom4.js• Put a link into the head:• <script type=“text/javascript” language=“javascript”
src=“javascriptdom4.js” />• Put checks into the code to ensure that your page
gracefully degrades if the user’s browser does not have sufficient level of support for Javascript
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innerHTML• Hands On• In the body of a blank HTML page insert a div
tag:<div id=“test”>This will be replaced</div>
• In the head of the page place this code:window.onload = function() {var testdiv = document.getElementBy Id(“testdiv”);testdiv.innerHTML = “<p>Now we have inserted
<em>this</em> instead!</p>”;}
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Using innerHTML
• All the HTML in the tag is replaced when the innerHTML method is used
• innerHTML is not part of the DOM – so it may one day disappear – though it is universally recognised by browsers
• Tags within the innerHTML are not part of the DOM tree so they cannot be manipulated
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Recap• Hands On• Now let us use the DOM to insert the same HTML into the div
tag1. Create an element node “p” assigned to the variable para2. Create a text node assigned to the variable txt1 (‘Now we
have inserted’)3. Append txt1 to para4. Create an element node em assigned to the variable
emphasis5. Append emphasis to para6. Create a text node assigned to the variable txt2 (‘this’)7. Append txt2 to emaphasis8. Append emphasis to para9. Create a text node assigned to the variable txt3 (‘instead!’)10.Append txt3 to para11.Append para to the element testdiv in the document
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Javascript and CSS
• Hands On• Open file JavascriptDOM6.html and
examine the code• Now try out each of the 3 user events• What do you notice about the difference
between appendChild and insertBefore?Syntax of insertBefore:
parentElement.insertBefore(newElement, targetElement)
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Points about CSS
• If style is set inline, it can be manipulated using Javascript
• If style is set by CSS, Javascript cannot directly manipulate it
• However, Javascript can set the style of the element and override the CSS
• Also, Javascript can indirectly manipulate style using the class tag
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Changing the Class Attribute• A useful way of manipulating style through Javascript is
to add a second class to an element eg:thiselement.className += “ newclass”;– adds the class ‘newclass’ to the class attribute
• Hands On• Open file JavascriptDOM7.html• Create a style for the tags with the class plain• Create a further style with more emphasis called
‘emphatic’• Write Javascript code that adds the emphatic class to an
element as the user mouseovers and removes it when the user mouseouts
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Objectives
• Understand the nature and structure of the DOM
• Add and remove content from the page • Access and change element attributes –
including source and class• Insert markup into a page using
innerHTML• Change style attribute using Javascript
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