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Javascript DOM

Javascript DOM. Peter Atkinson. 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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Javascript DOM

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  1. Javascript DOM Peter Atkinson

  2. 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

  3. 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

  4. Nodes Organise the Page <html> <head> <title>My page</title> </head> <body> <p>This is text on my page</p> </body> </html>

  5. What is a Node? • Element Node – contains an HTML tag • Text Node – contains text • Text Nodes are contained in Element Nodes

  6. Adding Some Text To A Page There are five steps: • Create a new Element • Create new Text • Append the new Text to the new Element • Find an existing Element • Append the new Element to the existing Element

  7. 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”)

  8. 2. Create a Text Node Next, create a text node Again, for convenience, we can put the new text node into a variable var newText; newText = document.createTextNode(“Some text.”)

  9. 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);

  10. 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”);

  11. 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);

  12. Putting the 5 Steps Together Hands 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>

  13. 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 On Modify your HTML page so that the user can click on some text to remove the text that was inserted

  14. 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

  15. 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?

  16. 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 On Add this code to JavascriptDOM1.html Execute the code

  17. Where on the Node Tree? • childNodes • nodeList = node.childNodes • firstChild • reference = node.firstChild • lastChild • nextSibling • parentNode • previousSibling

  18. 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()

  19. 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’?

  20. 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>

  21. 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

  22. 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

  23. 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

  24. 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;"

  25. Javascript for Functionality Graceful Degradation Progressive Enhancement CSS for Presentation HTML for Markup Web Page Architecture • Graceful Degradation = your site is navigable by users who do not have Javascript • Progressive Enhancement = page built in layers: The page should be constructed using 3 different files – one for each layer

  26. 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> Graceful Degradation

  27. 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

  28. 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

  29. 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”); }

  30. 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

  31. 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

  32. 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>”; }

  33. 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

  34. Recap • Hands On • Now let us use the DOM to insert the same HTML into the div tag • Create an element node “p” assigned to the variable para • Create a text node assigned to the variable txt1 (‘Now we have inserted’) • Append txt1 to para • Create an element node em assigned to the variable emphasis • Append emphasis to para • Create a text node assigned to the variable txt2 (‘this’) • Append txt2 to emaphasis • Append emphasis to para • Create a text node assigned to the variable txt3 (‘instead!’) • Append txt3 to para • Append para to the element testdiv in the document

  35. 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)

  36. 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

  37. 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

  38. 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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