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15.9.3 Form and Element Event Handlers

15.9.3 Form and Element Event Handlers


Each Form element has an onsubmit event handler to detect form submission and an onresetevent handler to detect form resets. The onsubmithandler is invoked just before the form is submitted; it can cancel the submission by returning false. This provides an opportunity for a JavaScript program to check the user’s input for errors in order to avoid submitting incomplete or invalid data over the network to a server-side program. Note that the onsubmit handler is triggered only by a genuine click on a Submit button. Calling the submit() method of a form does not trigger the onsubmit handler.

The onreset event handler is similar to the onsubmit handler. It is invoked just before the form is reset, and it can prevent the form elements from being reset by returning false. Reset buttons are rarely necessary in forms, but if you have one, you might want to make the user confirm the reset:

onreset="return confirm('Really erase ALL input and start over?')">

...

Clear and Start Over


Like the onsubmit handler, onreset is triggered only by a genuine Reset button. Calling the reset() method of a form does not trigger onreset.

Form elements typically fire a click or change event when the user interacts with them, and you can handle these events by defining an onclick or onchange event handler. The third column of Table 15-1 specifies the primary event handler for each form element. In general, form elements that are buttons fire a click event when activated (even when this activation happens through the keyboard rather than via an actual mouse click). Other form elements fire a change event when the user changes the value represented by the element. This happens when the user enters text in a text field or selects an option from a drop-down list. Note that this event is not fired every time the user types a key in a text field. It is fired only when the user changes the value of an element and then moves the input focus to some other form element. That is, the invocation of this event handler indicates a completed change. Radio buttons and checkboxes are buttons that have a state, and they fire both click and change events; the change event is the more useful of the two.

Form elements also fire a focus event when they receive keyboard focus and a blur event when they lose it.

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