JavaScript: The Definitive Guide, Fourth Edition

Name

EventListener — an event handler function DOM Level 2 Events

Availability

Methods

handleEvent( )

In languages such as Java that do not allow functions to be passed as arguments to other functions, you define an event listener by defining a class that implements this interface and includes an implementation for this handleEvent( ) method. When an event occurs, the system calls this method and passes in an Event object that describes the event.

In JavaScript, however, you define an event handler simply by writing a function that accepts an Event object as its argument. The name of the function does not matter, and the function itself is used in place of an EventListener object. See the Section section.

Description

This interface defines the structure of an event listener or event handler. In languages such as Java, an event listener is an object that defines a method named handleEvent( ) that takes an Event object as its sole argument. In JavaScript, however, any function that expects a single Event argument, or a function that expects no argument, can serve as an event listener.

Example

// This function is an event listener for a "submit" event
function submitHandler(e) {
  // Call a form-validation function defined elsewhere
  if (!validate(e.target))
      e.preventDefault(  );  // If validation fails, don't submit form
}

// We might register the event listener above like this
document.forms[0].addEventListener("submit", submitHandler, false);

See Also

Event, EventTarget; Chapter 19

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