How ajax works and how asynchronous requests are encapsulated

Ajax principles:
The client sends asynchronous requests through the built-in object XMLHttpRequest in the browser. When the server returns an response, it calls the previously registered callback function. In the callback function, you can use javascript to operate DOM to update the page, asynchronous requests do not block client operations, so that data can be updated if the page cannot be refreshed.
Encapsulation of asynchronous requests:
Copy codeThe Code is as follows:
Var xhr = false;
// Step 1: Create an XMLHttpRequest object compatible with different browser versions
If (window. XMLHttpRequest) {// IE7 +, Firefox, Chrome, Opera, Safari
Xhr = new XMLHttpRequest ();
} Else {
If (window. ActiveXObject) {// IE browser
Xhr = new ActiveXObject ("Microsoft. XMLHTTP"); // IE5 +
}
}
// Step 2: Set the callback function
Xhr. onreadystatechange = myCallback;
// Step 3: Create an asynchronous request
Xhr. open ("method", "url", true );
// If it is post: you need to set the encoding method for the request message body data.
Xhr. setRequestHeader ("Content-Type", "application/x-www-form-urlencoded ");
// Step 4: Send an asynchronous request
Xhr. send (content); // If the get method content is null, if it is post, the content is a "name = value" pair.