jquery when method

jQuery is a JavaScript library widely used in web development. It has efficient and fast DOM operation and event processing features, as well as portable AJAX operation capabilities. The when method is an asynchronous operation processing method based on Promise, which can realize the synchronous processing of multiple asynchronous operations. This article will introduce the use of jquery when method and common application scenarios.

1. Basic syntax of jquery when method
When we need to perform multiple asynchronous operations and proceed to the next step after they are completed, we can use the jquery when method. This method can reduce complex asynchronous processing logic and corresponding callback function nesting by creating a Promise object and returning the object.

When all asynchronous operations are successful, the parameters in the callback function will be an array composed of the results of each asynchronous operation; if an asynchronous operation fails, the callback function will return the failure reason of the asynchronous operation. This method can accept any number of asynchronous operation objects or iterable objects as parameters.

$.when(Asynchronous operation 1, Asynchronous operation 2, …).done(

function(异步操作1结果, 异步操作2结果, …) {
    // 在操作1和操作2都成功结束后执行的代码块
}

).fail(

function(错误原因) {
    // 当其他操作失败时执行的代码块
}

);

二, Common usage scenarios of the jquery when method

1. Requires multiple asynchronous requests to be completed before performing the next operation
   Sending multiple AJAX requests at the same time is a common use case in web development. In order to prevent each request from triggering the processing of the next request after receiving the response, we can use the jquery when method to wait for them all to complete. For example:

var promise1 = $.ajax({

url: "https://jsonplaceholder.typicode.com/todos/1",
method: "GET"

});

var promise2 = $.ajax({

url: "https://jsonplaceholder.typicode.com/todos/2",
method: "GET"

});

$.when(promise1, promise2).done(function(response1, response2) {

console.log(response1[0]); // 输出第一个Ajax响应数据
console.log(response2[0]); // 输出第二个Ajax响应数据

});

2. Check at any stage Is the data ready?
   During asynchronous data loading, we usually need to display a loading indicator or prevent users from performing meaningless operations. The jquery when method can easily implement this function, for example:

var callback = function() {

console.log("Do other stuff after data is loaded");

};

$.when($.ajax ('/url/to/resource'))

.done(function(response) {
    console.log("Data loaded successfully");
})
.always(callback);

3. Wait for multiple animations to complete before performing the next step
   We can encapsulate the completion event of the animation into a Promises object (multiple objects together) to wait for all animations to complete before performing the next step, such as:

var animation1 = $elem1.animate({ ... }).promise();
var animation2 = $elem2.animate({ ... }).promise();
var animation3 = $elem3.animate({ ... }).promise();

$ .when(animation1, animation2, animation3).done(function() {

console.log("All animations have completed");

});

Of course, we can also encapsulate multiple animations on the same element into a promise Object, such as:

var promisesArray = [];

promisesArray.push($element1.delay(1000).animate({ ... }).promise());
promisesArray.push($element1.fadeOut().promise());
promisesArray.push($element2.slideUp().promise());

$.when.apply(null , promisesArray).done(function() {

console.log("All animations have completed");

});

In short, the jquery when method is a synchronous processing solution suitable for a variety of asynchronous operations. The code is simple and easy to manage. , so it is widely used in actual development. I hope this article can provide readers with some enlightenment about the basic syntax and common application scenarios of the jquery when method.

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