Scripting Event Handlers

JavaScript applications in the Navigator are largely event-driven. Events are actions that occur, usually as a result of something the user does.

For example, a button click is an event, as is giving focus to a form element. There is a specific set of events that Navigator recognizes. You can define Event handlers, scripts that are automatically executed when an event occurs.

Event handlers are embedded in documents as attributes of HTML tags to which you assign JavaScript code to execute. The general syntax is

<TAG eventHandler="JavaScript Code"> where TAG is some HTML tag and eventHandler is the name of the event handler.

For example, suppose you have created a JavaScript function called compute. You can cause Navigator to perform this function when the user clicks on a button by assigning the function call to the button's onClick event handler:

You can put any JavaScript statements inside the quotes following onClick. These statements get executed when the user clicks on the button. If you want to include more than one statement, separate statements with a semicolon (;).

In general, it is a good idea to define functions for your event handlers because:

it makes your code modular-you can use the same function as an event handler for many different items.
it makes your code easier to read.

Notice in this example the use of this.form to refer to the current form. The keyword this refers to the current object-in the above example, the button object. The construct this.form then refers to the form containing the button. In the above example, the onClick event handler is a call to the compute() function, with this.form, the current form, as the parameter to the function.

Events apply to HTML tags as follows:

Focus, Blur, Change events: text fields, textareas, and selections
Click events: buttons, radio buttons, checkboxes, submit buttons, reset buttons, links
Select events: text fields, textareas
MouseOver event: links

If an event applies to an HTML tag, then you can define an event handler for it. In general, an event handler has the name of the event, preceded by "on." For example, the event handler for the Focus event is onFocus.

Many objects also have methods that emulate events. For example, button has a click method that emulates the button being clicked. Note: The event-emulation methods do not trigger event-handlers. So, for example, the click method does not trigger an onClick event-handler. However, you can always call an event-handler directly (for example, you can call onClick explicitly in a script).

Event	Occurs when...	Event Handler
blur	User removes input focus from form element	onBlur
click	User clicks on form element or link	onClick
change	User changes value of text, textarea, or select element	onChange
focus	User gives form element input focus	onFocus
load	User loads the page in the Navigator	onLoad
mouseover	User moves mouse pointer over a link or anchor	onMouseOver
select	User selects form element's input field	onSelect
submit	User submits a form	onSubmit
unload	User exits the page	onUnload

Example 4: a script with a form and an event handler attribute.

<HEAD> <SCRIPT LANGUAGE="JavaScript"> function compute(form) { if (confirm("Are you sure?")) form.result.value = eval(form.expr.value) else alert("Please come back again.") } </SCRIPT> </HEAD> <BODY> <FORM> Enter an expression: <INPUT TYPE="text" NAME="expr" SIZE=15 > <INPUT TYPE="button" VALUE="Calculate" ONCLICK="compute(this.form)"> <BR> Result: <INPUT TYPE="text" NAME="result" SIZE=15 > <BR> </FORM> </BODY>

Example 4 page display.

Using Quotes

Be sure to alternate double quotes with single quotes. Since event handlers in HTML must be enclosed in quotes, you must use single quotes to delimit arguments. For example

Alternatively, you can escape quotes by preceding them by a backslash (\).