How to call callback function in jquery

When writing jQuery code, callback functions are a very common programming method. The callback function is automatically called under specific conditions. This method is very suitable for handling asynchronous requests and event responses. In this article, we will explore in detail how to call jQuery callback functions.

1. What is a callback function?

Callback function refers to a function that is passed to another function (that is, the parent function). The function passed in can be executed synchronously or asynchronously.

In JavaScript, callback functions are often used in scenarios such as asynchronous requests and event processing. When the event or asynchronous call is completed, the result is returned to the user through the callback function.

2. Basic usage of jQuery callback function

In jQuery, the callback function can be passed as a parameter to different methods. For example, when we use jQuery to initiate an Ajax request, we can pass the callback function as a parameter to the request. When the request is completed, the callback function will be automatically called.

For example, the following is a simple $.get() request example, through which you can clearly see that the callback function is passed as a parameter to the $.get method.

$.get('http://url.com', function(data) {
    console.log(data);
});

In the above example, when the $.get request is completed, the callback function of the request will be called, and the obtained data will be passed to the callback function as a parameter.

3. Use Deferred objects to implement callback functions

In addition to simple callback function definitions, jQuery also provides a powerful tool, the Deferred object, which can handle asynchronous requests more conveniently. and callback function.

We regard jQuery's Deferred object as a layer of encapsulation of the callback function to achieve more flexible control and greater scalability. The following is a simple Deferred object example:

var deferred = $.Deferred();

$.get('http://url.com').done(function(data) {
    deferred.resolve(data);
}).fail(function() {
    deferred.reject();
});

deferred.promise().done(function(data) {
    console.log(data);
});

In the above example, a Deferred object is first created, and then two callback functions done (success) and fail (failure) are registered with this object.

When the request is successful, use the resolve() function to return the result to the deferred object. If it fails, call the reject() function.

Finally, obtain the ajax request promise object through the promise() function, and return the result to the user through the done() function.

In addition to done() and fail(), Jquery provides some other Promise object callback functions:

• always(callbacks): Add a js callback function in the Deferred object Will be executed regardless of success or failure.
• catch(callbacks): Add a js callback function that is called when a Deferred object is rejected.
• then(callbacks, [callbacks], [callbacks]): Add one or more callbacks to the current object. These callbacks will be called in the resolve() and reject() callbacks of Deferred.

The above is the key point, remember to flexibly use and combine the coding methods with specific scenes.

4. Another way to call the callback function - event

Another way to call the callback function is through an event. We can manually trigger the event in code and pass the callback function as an event handler to the event.

For example, the following is a simple click event example:

$('button').on('click', function() {
    console.log('Button was clicked!');
});

In the above example, when the user clicks the button, the click event will be triggered, and the callback function of the event will output a button Clicked information.

5. Summary

Callback functions and events are very common programming methods in jQuery and are widely used in code. The callback function can be called by passing parameters, using Deferred objects and events, and has the characteristics of high flexibility and strong scalability.

When writing jQuery code, understanding the basic usage of callback functions and the use of Deferred objects can improve our programming efficiency and code quality.

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