Event handling in JavaScript

Overview of Event Processing
Event processing is a very important part of object-based programming. Without event processing, the program will become dead and lack flexibility. The process of event handling can be expressed like this: an event occurs - the event handler is started - the event handler reacts. Among them, in order for the event handler to be started, the object must first be told what handler to start if something happens, otherwise the process cannot continue. The event handler can be any JavaScript statement, but we usually use a specific custom function to handle things.
There are three ways to specify event handlers:
Method 1 Specify directly in HTML tags. This method is the most commonly used. The method is:

Let's look at an example:

This definition of the tag can cause a dialog box to pop up when the document is read, saying "The web page has been read, please enjoy it slowly"; when the user exits "Goodbye" pops up when opening the document (either closing the window or going to another page).
Method 2: Write JavaScript for specific events of specific objects. This method is used less often, but it is still useful in some situations. The method is:


Method 3 is explained in JavaScript. Method:
. = ;
The thing to note with this method is that the "event handler" is a real code, not a string form code. If the event handler is a custom function, do not add "()" if there is no need to use parameters.
function ignoreError() {
return true;
}
window.onerror = ignoreError; // No "()" is used
This example defines the ignoreError() function as the handler for the onerror event of the window object. Its effect is to ignore any errors under the window object ("no permission" errors caused by referencing a location object that does not allow access cannot be ignored).

Detailed explanation of the event
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onblur event occurs when the window loses focus. Applies to: window object

onchange event occurs after the content of the text input area is changed and then the focus is moved from the text input area. Capturing this event is mainly used to detect the validity of input in real time, or to change the document content immediately. Applies to: Password object; Select object; Text object; Textarea object

onclick event occurs when the object is clicked. Clicking refers to the complete process of hovering the mouse on an object, pressing the mouse button, and releasing the mouse button without moving the mouse. A normal button object (Button) usually has an onclick event handler, because this kind of object cannot get any information from the user at all, and it is useless without an onclick event handler. Adding an onclick event handler to the button can simulate "another submit button" by changing one or several attributes of the form such as action, target, encoding, method, etc. in the event handler, and then calling the submit() method of the form. . Returning false in the onclick event handler of the Link object prevents the browser from opening this connection. That is, if there is a connection like this: http://www.a.com" false">Go!, Then no matter how the user clicks, they will not go to the www.a.com website unless the user prohibits the browser from running JavaScript. Applies to: Button object; Checkbox object; Image object; Link object; Radio object; Reset object; Submit object

onerror event occurs when an error occurs. Its event handler is usually called an "Error Handler" and is used to handle errors. As mentioned above, to ignore all errors, use:
function ignoreError() {
return true;
}
window.onerror = ignoreError;
Apply to: window object

onfocus event occurs when the window gets focus. Applies to: window object

onload event occurs when all documents are downloaded. All downloads have been completed, which means that not only the HTML files, but also all included images, plug-ins, controls, applets, etc. have been downloaded. This event is a window event, but when specifying the event handler in HTML, we write it in the tag. Applies to: window object

event occurs when the user places the mouse on the object and presses the mouse button. Reference events. Applies to: Button object; Link object

event occurs when the mouse leaves the object. Reference events. Applies to: Link object

event occurs when the mouse enters the object range. This event and the event, coupled with the pre-reading of the image, can achieve the effect of the image changing when the mouse moves over the image connection. Sometimes we see that when pointing to a connection, the address is not displayed on the status bar, but other information is displayed. It seems that these information can be changed at any time. This is how they are made:
seover="window.status=Click Me Please!; return true;" onmouseout="window.status= ; return true;">
Applies to: Link object

The event occurs when the user places the mouse on the object and the mouse button is pressed, and then releases the mouse button. If the mouse button is pressed while the mouse is not on the object where the mouse was released, this event will not occur. Applies to: Button object; Link object

onreset event occurs when the "Reset" button of the form is clicked (pressed and released). You can prevent the form from resetting by returning false in the event handler. Applies to: Form object

onresize event occurs when the window is resized. Applies to: window object

onsubmit event occurs when the "Submit" button of the form is clicked (pressed and released). You can use this event to verify the validity of the form. Form submission can be prevented by returning false in the event handler. Applies to: Form object

onunload event occurs when the user exits the document (or closes the window, or goes to another page). Like onload, if you want to write it in HTML, write it in the tag. Some Web Masters use this method to pop up a "survey form" to "force" visitors to fill in; some pop up advertising windows to instigate visitors to click on the link. I think this "onunload="open..."" method is very bad. Sometimes it even causes a lack of resources because too many windows pop up. If you have anything to say to the visitor, you should say it on the web page, right? Applies to: window object