Event handling in JavaScript_Basic knowledge

Overview of event processing
Event processing is a very important part of object-based programming. Without event processing, the program will become very dead and lack flexibility. The process of event processing 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 generally 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 take a look at an example:

Such a definition of the tag will allow the document to be read when it is finished A dialog box pops up saying "The web page has been read, please enjoy it slowly"; "Goodbye" pops up when the user exits the document (or closes the window, or goes 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: .
=
; <script> <BR>... <BR>(事件处理程序代码) <BR>... <BR></script><script> <BR> alert('网页读取完成，请慢慢欣赏！'); <BR></script>The thing to note with this method is that the "event handler" is the real code, not the code in the form of a string. 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; // "()" is not 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).

Event details



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 more attributes of the form such as action, target, encoding, and method in the event handler, and then calling the form's submit() method. . Returning false in the onclick event handler of the Link object prevents the browser from opening this connection. That is, if there is a link like this: 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 The object

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

onload event occurs when all documents are downloaded. The completion of all downloads means that not only the HTML files, but also all the 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

onmousedown event occurs when the user places the mouse on the object and presses the mouse button. Refer to the onmouseup event. Applies to: Button object; Link object

onmouseout event occurs when the mouse leaves the object. Refer to the onmouseover event. Applies to: Link object

onmouseover event occurs when the mouse enters the object range. This event and the onmouseout event, coupled with the pre-reading of the image, can achieve the effect of changing the image 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. They are made like this:


Applies to: Link object

The onmouseup event occurs when the user places the mouse on the object and the mouse button is pressed. when releasing 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). The form reset can be prevented 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 "survey forms" 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 visitors, you should say it on the web page, right? Applies to: window object