Understanding variable scopes in the success callback function of a JQuery post request

I am encountering an issue with passing the AJAX success array from one function to another. It seems that I am unable to transfer the data stored in a variable within the success section of the AJAX function to the parent function for return.

I tried to find a solution by referring to this post, but unfortunately, I couldn't make sense of it: jQuery Ajax call - Set variable value on success

I would greatly appreciate any help or guidance on this matter.

Below is a simplified version of the code:

// declare json_to_return as a global variable
var json_to_return;

function loop_through_data(){

  // invoke the load_days function and store its array data in days_array
  var days_data = load_days(03,2010);

  // my goal is to iterate through days_data at this point
}

function load_days(selectedMonth, selectedYear){  
                  $.ajax({
                   type: "POST",
                   dataType: "json",
                   url: "../includes/get_availability.php",
                   data: "month=" + selectedMonth + "&year=" + selectedYear,
                   success: function(available_json){
                       json_to_return = available_json;
                   },
                   error: function(msg){
                    alert("error " + msg);
                   }
                 });
               return json_to_return;

}   

Answer №1

This specific portion of your function will be executed at a later point:

success: function(available_json){
                   json_to_return = available_json;
               }

As a result, the variable you are attempting to return is currently undefined because the related code hasn't been processed yet - it will only be set after receiving a response from the server. You have two options: either trigger the remaining code from within your success function to ensure its execution when the data is available, or utilize async:false (though this approach may not be optimal as it can impact browser performance).

The async: false technique involves the following:

              $.ajax({
               type: "POST",
               async: false,
               dataType: "json",
               url: "../includes/get_availability.php",
               data: "month=" + selectedMonth + "&year=" + selectedYear,
               success: function(available_json){
                   json_to_return = available_json;
               },
               error: function(msg){
                alert("error " + msg);
               }
             });

Hello there.

              $.ajax({
               type: "POST",
               dataType: "json",
               url: "../includes/get_availability.php",
               data: "month=" + selectedMonth + "&year=" + selectedYear,
               success: function(available_json){
                   loopThroughTheDaysAndDoStuff(available_json);
               },
               error: function(msg){
                alert("error " + msg);
               }
             });

Answer №2

$.ajax is a function that works asynchronously.

When this function is called, it continues to execute without waiting for the result.

In the code you provided

var days_data = load_days(03,2010);
, the following sequence of events occurs:

  • An ajax call begins (asynchronous, function keeps executing)
  • Returns json_to_return (still undefined at this point)
  • days_data is still undefined
  • Ajax call completes, and finally json_to_return gets assigned (this timing can vary)

You need to reconsider your approach and write your code with consideration for the asynchronous nature of the ajax call.

One possible solution is to pass a callback function to load_days, which will be executed after the successful completion of the ajax call.

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