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JavaScript and HTML. Simple Event Handling. JavaScript and DOM. JavaScript relies on a Document Object Model ( DOM ) that describes the structure of the web page This is not the same as the XML DOM You can do a lot with a just a little understanding of the DOM

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javascript and html

JavaScript and HTML

Simple Event Handling

javascript and dom
JavaScript and DOM
  • JavaScript relies on a Document Object Model (DOM) that describes the structure of the web page
    • This is not the same as the XML DOM
  • You can do a lot with a just a little understanding of the DOM
    • You use the DOM to access elements on the web page
    • You can capture events without knowing the DOM at all
    • You need the DOM to make any changes to the web page
  • Some (but not all) elements on the web page respond to user interactivity (keystrokes, mouse clicks) by creating events
    • Different kinds of elements produce different events
      • Browsers are not all alike in what events are produced
    • We will concentrate on events from HTML form elements and commonly recognized events
  • You can put handlers on HTML form elements
    • If the event isn’t generated, the handler does nothing
    • A handler should be very short
      • Most handlers call a function to do their work
a simple event handler
A simple event handler
  • <form method="post" action=""> <input type="button" name="myButton" value="Click me" onclick="alert('You clicked the button!');"></form>
    • The button is enclosed in a form
    • The tag is input type="button"
    • The name can be used by other JavaScript code
    • The value is what appears on the button
    • onclick is the name of the event being handled
      • The value of the onclick element is the JavaScript code to execute
      • alert pops up an alert box with the given text
  • JavaScript is case sensitive
  • HTML is not case sensitive
  • onclick="alert('You clicked the button!');"
    • The underlined parts are HTML
    • The quoted string is JavaScript
    • You will frequently see onclick capitalized as onClick
      • The Java naming convention is easier to read
      • This is fine in HTML, but an error if it occurs in JavaScript
  • Also note: Since we have a quoted string inside another quoted string, we need both single and double quotes
common events
Common events
  • Most HTML elements produce the following events:
    • onClick -- the form element is clicked
    • onDblClick -- the form element is clicked twice in close succession
    • onMouseDown -- the mouse button is pressed while over the form element
    • onMouseOver -- the mouse is moved over the form element
    • onMouseOut -- the mouse is moved away from the form element
    • onMouseUp -- the mouse button is released while over the form element
    • onMouseMove -- the mouse is moved
  • In JavaScript, these should be spelled in all lowercase
example simple rollover
Example: Simple rollover
  • The following code will make the text Hellored when the mouse moves over it, andblue when the mouse moves away

<h1 onMouseOver="style.color='red';" onMouseOut="style.color='blue';">Hello </h1>

  • Image rollovers are just as easy:

<img src="../Images/duke.gif" width="55" height="68" onMouseOver="src='../Images/duke_wave.gif';" onMouseOut="src='../Images/duke.gif';">

events and event handlers i
Events and event handlers I
  • The following tables are taken from: jsguide/index.htm
back to the dom
Back to the DOM
  • You can attach event handlers to HTML elements with very little knowledge of the DOM
  • However, to change what is displayed on the page requires knowledge of how to refer to the various elements
  • The basic DOM is a W3C standard and is consistent across various browsers
    • More complex features are browser-dependent
  • The highest level element (for the current page) is window, and everything else descends from that
    • Every JavaScript variable is a field of some object
    • In the DOM, all variables are assumed to start with “window.”
    • All other elements can be reached by working down from there
the dom hierarchy
The DOM hierarchy


fields of window i
Fields of window, I
  • window
    • The current window (not usually needed).
  • self
    • Same as window.
  • parent
    • If in a frame, the immediately enclosing window.
  • top
    • If in a frame, the outermost enclosing window.
  • frames[ ]
    • An array of frames (if any) within the current window. Frames are themselves windows.
  • length
    • The number of frames contained in this window.
fields of window ii
Fields of window, II
  • document
    • The HTML document being displayed in this window.
  • location
    • The URL of the document being displayed in this window. If you set this property to a new URL, that URL will be loaded into this window. Calling location.reload()will refresh the window.
  • navigator
    • A reference to the Navigator (browser) object. Some properties of Navigator are:
      • appName -- the name of the browser, such as "Netscape"
      • platform -- the computer running the browser, such as "Win32"
  • status
    • A read/write string displayed in the status area of the browser window. Can be changed with a simple assignment statement.
methods of window i
Methods of window, I
  • alert(string)
    • Displays an alert dialog box containing the string and an OK button.
  • confirm(string)
    • Displays a confirmation box containing the string along with Cancel and OK buttons. Returns true if OK is pressed, false if Cancel is pressed.
  • prompt(string)
    • Displays a confirmation box containing the string, a text field, and Cancel and OK buttons. Returns the string entered by the user if OK is pressed, null if Cancel is pressed.
methods of window ii
Methods of window, II
  • open(URL)
    • Opens a new window containing the document specified by the URL.
  • close()
    • Closes the given window (which should be a top-level window, not a frame).
fields of document i
Fields of document, I
  • You must prefix these fields with document.
  • anchors[ ]
    • An array of Anchor objects (objects representing<a name=...> tags)
  • applets[ ]
    • An array of Applet objects
      • The properties are the public fields defined in the applet
      • The methods are the public methods of the applet
      • Cautions:
        • You must supply values of the correct types for the fields and method parameters
        • Changes and method calls are done in a separate Thread
fields of document ii
Fields of document, II
  • forms[ ]
    • An array of Formelements
      • If the document contains only one form, it is forms[0]
  • images[ ]
    • An array of Image objects
      • To change the image, assign a new URL to the src property
  • links[ ]
    • An array of Link objects
      • A link has several properties, including href, which holds the complete URL
fields of document iii
Fields of document, III
  • bgColor
    • The background color of the document
      • May be changed at any time
  • title
    • A read-only string containing the title of the document
  • URL
    • A read-only string containing the URL of the document
fields of the form object
Fields of the form object
  • elements[ ]
    • An array of form elements