This article explains how to cancel events defined by standard specifications in JavaScript .
I would like to know how to cancel an event defined in the standard specifications. How can I cancel an event?
To cancel a standard event, use event.preventDefault().
Summary for canceling an event
In JavaScript, an event is a mechanism that fires when a specific action is performed. Examples of events include mouse clicks, keyboard input, and form submissions.
These events can be handled with JavaScript code. For example, when a form is submitted, you can perform a process to validate the input contents.
However, sometimes you need to cancel the firing of an event . For example, you might want to validate input before submitting a form, and abort the form submission if there is a problem.
In these cases, you need a way to cancel the event. JavaScript has special methods and properties for canceling events. By using these, you can cancel the firing of an event.
Cancellation of an event must be done before the event is fired or within an event listener. Cannot be canceled after the event has occurred.
The sample code below is an example of validating the input contents when a form element’s submit event occurs, and canceling the event if there is a problem.
const form = document.querySelector('form');
form.addEventListener('submit', function(event) {
if (!validateForm()) {
event.preventDefault();
}
});
function validateForm() {
// 入力内容の検証を行う
// Returns false if there is a problem
}In this example, when the submit event occurs, validateFormthe function validates the input contents, and if there is a problem, event.preventDefault()the event is canceled.
Explanation of event cancellation using a sample program
We will provide a detailed explanation using a sample program for event cancellation.
The sample program below cancels the context menu that appears when you right-click the mouse. I will use this to explain how to cancel an event.
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Sample program for event cancellation</title>
</head>
<body>
<p>Try right-clicking.</p>
<script>
// Cancel default right-click behavior
document.addEventListener('contextmenu', function(event) {
event.preventDefault();
});
</script>
</body>
</html>The program specifies to the document object to listen for the contextmenu event. the contextmenu event is the event that occurs when the right mouse button is pressed.
Within this listener function, the default behavior of the event (in this case, the display of the contextmenu) is canceled by calling event.preventDefault(). In other words, when this program is executed, the context menu from a right mouse click will no longer be displayed.
Thus, by calling event.preventDefault(), the default behavior of the event can be canceled.
summary
We explained how to cancel events defined in the standard specifications.
- Sometimes it is necessary to cancel the firing of an event.
- The event can be canceled by using event.preventDefault().
- Event cancellation must be done within the event listener.
Now you know how to cancel a standard event!
Event cancellation is one of the very important features in web application development.
By making good use of event cancellation, you can eliminate user inconvenience and avoid security problems.
However, misuse of event cancellation can also seriously impact the behavior of your application.
Also, event cancellation may not work properly depending on the browser, so it is important to check that it works properly.
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