Difference between Synchronous and Asynchronous Ajax request - ajax

Question

Other than that if there is some code after ajax call In asynchronous it executes first. In synchronous It executes one after the other in the code.

Answer 1

Synchronous ( async: false ) – Script stops and waits for the server to send back a reply before continuing. There are some situations where Synchronous Ajax is mandatory. In standard Web applications, the interaction between the customer and the server is synchronous. This means that one has to happen after the other.
Synchronous part:
$.ajax({
url: "file.php",
type: "POST",
async: false,
success: function(data) {
// write something awesome in response data part
}
});
Asynchronous Part:
$.ajax({
url: "file.php",
type: "POST",
async: true,
success: function(data) {
// write something awesome in response data part
}
});
Thats it. Basically people use asynchronous when they need to send request on completion of first request. Means, your requests will work one by one not all request will send at once. Instead of it, in synchronous ajax calls your all requests will be done synchronously.

Answer 2

Basically, Asynchronous requests do not really mind other codes if they have already executed or not. It only executes if it has already received the response from where you sent the request.
Ajax requests are Asynchronous by nature, but it can be set to Synchronous, thus, having the codes before it, execute first.
// Note: Synchronous ajax request already deprecated by jquery, it's better to use promises.

Answer 3

Synchronous Ajax request: It blocks event loop, meaning you/browser cannot do other activities (clicking a button, loading images etc )
Asynchronous Ajax request: It doesn't block event loop and able to do other activities while ajax is processing.
Event loop: Mechanism in javascript to achieve asynchronous operations. For details you can google it.

Answer 4

That's right.
Examples for Javascript:
var xhr = new XMLHttpRequest();
var button = document.getElementById('button');
xhr.open('GET', '/', true);
xhr.send(); // (1)
xhr.onreadystatechange = function() { // (3)
if (xhr.readyState != 4) return;
button.innerHTML = 'Ready!';
if (xhr.status != 200) {
alert(xhr.status + ': ' + xhr.statusText);
} else {
alert(xhr.responseText);
}
}
button.innerHTML = 'Loading...'; // (2)
button.disabled = true;
If there is third argument in xhr.open() and it equals true, then request will be asynchronous. It means that after call xhr.send() the code at (1) doesn't «hang», but still executes, the string (2) executes, and result of xhr request obtains after state-change event (3).
I slightly changed the code example from this article:
https://javascript.info/xmlhttprequest

Answer 5

<h2>Synchronous Request</h2>
<p id="demo"></p>
<script>
var xmlhttp = new XMLHttpRequest();
alert("after creating XMLHttpRequest ");
xmlhttp.onreadystatechange = function () {
if (this.readyState == 4 && this.status == 200) {
var myObj = JSON.parse(this.responseText);
alert("In function");
document.getElementById("demo").innerHTML = "Name from JSON Data : " + myObj.name;
}
};
xmlhttp.open("GET", "emp.json", false);//Synchronous
xmlhttp.send();
alert("after send");
</script>
For better understanding I used alert to know how synchronous and asynchronous requests takes place,set as true and see