Experience summary of JavaScript asynchronous requests and data processing in front-end development

In front-end development, JavaScript is a very important language. It can not only realize the page Interactive and dynamic effects, and data can also be obtained and processed through asynchronous requests. In this article, I will summarize some experiences and tips when dealing with asynchronous requests and data.

1. Use the XMLHttpRequest object for asynchronous requests
The XMLHttpRequest object is JavaScript’s standard method for sending and receiving HTTP requests. When processing asynchronous requests, you can send requests through this object and monitor its status changes. The following is a simple example of using the XMLHttpRequest object to send a GET request:

var xhr = new XMLHttpRequest();
xhr.open("GET", "http://example.com/api/data", true);
xhr.onreadystatechange = function() {
  if (xhr.readyState === 4 && xhr.status === 200) {
    var data = JSON.parse(xhr.responseText);
    // 處理返回的數(shù)據(jù)
  }
};
xhr.send();

In the above code, we first create an XMLHttpRequest object, then call the open method to set the request method and URL, and pass in a Boolean A value of true indicates that the request is sent asynchronously. Next, we use the onreadystatechange event listener to monitor the status changes of the request. When the readyState of the request is 4 and the status code is 200, it means that the request is successful, and the returned data can be obtained through the responseText attribute. Next, we can convert the returned data into a JavaScript object through the JSON.parse method and perform further processing.

2. Use Fetch API for asynchronous requests
In addition to the XMLHttpRequest object, modern browsers also provide a new Fetch API to handle asynchronous requests. The Fetch API provides more powerful and flexible functionality, supports Promise and friendlier syntax. The following is an example of using the Fetch API to send a GET request:

fetch("http://example.com/api/data")
  .then(function(response) {
    if (response.ok) {
      return response.json();
    } else {
      throw new Error("請(qǐng)求失敗");
    }
  })
  .then(function(data) {
    // 處理返回的數(shù)據(jù)
  })
  .catch(function(error) {
    // 處理異常
  });

In the above code, we call the fetch method and pass in the requested URL. The fetch method returns a Promise object, and the return data when the request is successful can be processed through the then method. In the then method, we first determine whether the status code of the request is 200. If so, call the response.json method to convert the returned data into a JavaScript object. Next, we can further process the returned data in the then method. If the request fails, an exception will be thrown, which can be handled by the catch method.

3. Data processing techniques
When processing data returned by asynchronous requests, there are some techniques that can improve the readability and performance of the code:

1. Use template strings
   Template string is a string literal that can contain variables and expressions. When processing the returned data, you can use template strings to splice data in HTML or other formats. For example:

var name = "John";
var age = 30;
var html = `<div>Name: ${name}</div><div>Age: ${age}</div>`;

2. Using arrow functions
   The arrow function is a new way of defining functions introduced in ES6. It can define functions more concisely and inherit the this value of the current context. . When processing the returned data, you can use arrow functions to handle data traversal and mapping. For example:

var users = [{ name: "John", age: 30 }, { name: "Mary", age: 25 }];
var names = users.map(user => user.name);

3. Avoid nesting callback functions too deeply
   Nesting callback functions too deeply will make the code difficult to read and maintain. When processing asynchronous requests, you can use Promise or async/await to avoid nesting callback functions too deeply. For example:

fetch("http://example.com/api/data")
  .then(function(response) {
    if (response.ok) {
      return response.json();
    } else {
      throw new Error("請(qǐng)求失敗");
    }
  })
  .then(function(data) {
    return data.filter(user => user.age > 18);
  })
  .then(function(filteredData) {
    // 處理過濾后的數(shù)據(jù)
  })
  .catch(function(error) {
    // 處理異常
  });

In the above code, we process data in series through multiple then methods instead of nesting multiple callback functions.

4. Conclusion
In front-end development, JavaScript asynchronous requests and data processing are essential parts. By mastering the use of the XMLHttpRequest object and Fetch API, and applying some data processing techniques, you can improve development efficiency and enhance user experience. I hope the above experience summary will be helpful to your asynchronous requests and data processing in front-end development.

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