Apr 21, 2023

JavaScript and HTML

Simple Event Handling

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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

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Events

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

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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

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Capitalization

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

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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

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Example: Simple rollover

The following code will make the text Hello red when the mouse moves over it, and blue 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';">

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Events and event handlers I

The following tables are taken from:http://developer.netscape.com/docs/manuals/js/client/ jsguide/index.htm

Event Applies to Occurs when Handler

Load Document body User loads the page in a browser

onLoad

Unload Document body User exits the page onUnload

Error Images, window Error on loading an image or a window

onError

Abort Images User aborts the loading of an image

onAbort

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Events and event handlers II

Event Applies to Occurs when Handler

KeyDown

Documents, images, links, text areas

User depresses a key

onKeyDown

KeyUp Documents, images, links, text areas

User releases a key

onKeyUp

KeyPress

Documents, images, links, text areas

User presses or holds down a key

onKeyPress

Change Text fields, text areas, select lists

User changes the value of an element

onChange

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Events and event handlers III

Event Applies to Occurs when Handler

MouseDown

Documents, buttons, links

User depresses a mouse button

onMouseDown

MouseUp Documents, buttons, links

User releases a mouse button

onMouseUp

Click Buttons, radio buttons, checkboxes, submit buttons, reset buttons, links

User clicks a form element or link

onClick

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Events and event handlers IV

Event Applies to Occurs when Handler

MouseOver

Links User moves cursor over a link

onMouseOver

MouseOut Areas, links User moves cursor out of an image map or link

onMouseOut

Select Text fields, text areas

User selects form element’s input field

onSelect

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Events and event handlers V

Event Applies to Occurs when Handler

Move Windows User or script moves a window

onMove

Resize Windows User or script resizes a window

onResize

DragDrop Windows User drops an object onto the browser window

onDragDrop

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Events and event handlers VI

Event Applies to Occurs when Handler

Focus Windows and all form elements

User gives element input focus

onFocus

Blur Windows and all form elements

User moves focus to some other element

onBlur

Reset Forms User clicks a Reset button

onReset

Submit Forms User clicks a Submit button

onSubmit

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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

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The DOM hierarchy

Source: http://sislands.com/coin70/week1/dom.htm

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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.

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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.

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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.

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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).

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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

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Fields of document, II

forms[ ] An array of Form elements

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

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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

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Fields of the form object

elements[ ] An array of form elements

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The End