Ajax implements synchronous and asynchronous requests

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Release： 2018-07-18 16:38:28

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ajax has synchronous requests and asynchronous requests. Synchronous requests mean that only one process is allowed to be executed at the same point in time. Asynchronous requests mean that multiple processes can be executed at the same point in time.

ajax object.open (method get/post, url address, [asynchronous true] synchronous false);

ajax can communicate with the server One of the techniques for conducting (asynchronous or synchronous) interactions.

Asynchronous: Allows the execution of multiple processes at the same point in time.

Synchronization: Only one process is allowed to be executed at the same point in time.

Server-side 04.php:

<?php

echo "computer";

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Synchronous request:

Client04async.html：

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>ajax同步</title>
	<script type="text/javascript">
	alert(1234);
	</script>
</head>
<body>
	<img src="./1.png" alt="" width="400" height="300">
</body>
</html>

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Achievement effect:

##Asynchronous request:

Client05async.html：

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>ajax同步</title>
	<script type="text/javascript">
		var xhr = new XMLHttpRequest();
		xhr.onreadystatechange = function(){
			if (xhr.readyState==4) {
				alert(xhr.responseText);
			}
		}
		xhr.open(&#39;get&#39;, &#39;./04.php&#39;, true);//异步请求
		//xhr.open(&#39;get&#39;, &#39;./04.php&#39;, false);//同步请求
		xhr.send(null);
	</script>
</head>
<body>
	<img src="./1.png" alt="" width="400" height="300">
</body>
</html>

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Achievement effect:

Asynchronous requests are only visually presented to the user at the same time, but if the ajax request response or img response time is inconsistent, there will also be a sequence, but Multiple processes are executed at the same time at the same time.

When to use synchronous requests?

Ajax performs asynchronous requests in most cases, but sometimes it is necessary to use "synchronous requests ” (It cannot be replaced).

For example, the page has two parts, the ajax request content and the normal html content output. If the html output content includes the ajax request content, you need to use the ajax request to complete the html content. For the output of content, it is necessary to set the two to be called one after the other (rather than at the same time), which requires synchronous request.

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