Is client-side JavaScript executed prior to server-side JavaScript in AJAX?


I'm curious about how client-side code interacts with server-side responses. I have a lengthy and complex piece of code that triggers a function which in turn executes some server-side code using a HTTPRequest Object, returning a string to the calling page. However, it seems like the client-side code is processing before the response is received, causing my code to fail. For instance, when I call my function with the HTTPRequest, the next line attempts to display the returned value in an alert, but the alert appears empty. A few lines later, the same alert displays the expected value. This has led me to believe that the initial client-side alert is rendering a string that hasn't been returned yet, while the second alert waits for the server to process the request before displaying the correct value. Is this explanation plausible? Will the client-side code run regardless of whether the HTTPRequest is completed? If so, are there any methods to prevent the client-side code from executing until a response is received from the server?

javascriptajaxclient-sideserver-side

Answer №1

When making an HTTP Request, the process is asynchronous. This means that your code will send the request and then continue with its execution without waiting for a response. To handle the received data from the XHR request, you must create a callback function to process it in some manner.

Answer №2

To properly handle the response from an XmlHttpRequest object, you must assign an anonymous function to its onreadystatechange event like this:

//Assuming xhr is a correctly instantiated instance of the XmlHTTPRequest object
xhr.onreadystatechange = function(){  //Check if the request has been completed successfully
  if (xhr.readyState === 4){
    //Place your code logic here
  }
}

Answer №3

In order to ensure that your code properly handles the returned value from an AJAX request, it is important to make sure that the code is contained within the ajax routine's scope. This guarantees that the response will not be processed until the request has been fully resolved. For instance, if you are utilizing jQuery: ACCURATE

jQuery.get('ajax/ajax.getSomeData.php',{id:id}, function(data) {
    $('#myDIV').html(data);
});

INCORRECT APPROACH

jQuery.get('ajax/ajax.getSomeData.php',{id:id}, function(data) {});
$('#myDIV').html(data);

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