Confused about the concept of an Ajax callback? Let's break it down

Question

Confused about the concept of an Ajax callback? Let's break it down

I am retrieving data from a file, specifically /notes.html. I am attempting to save the result of the ajax call in a variable, but for some reason it keeps showing up as undefined. However, when I execute the function in chrome developer tools, it displays the content of /notes.html. Below is my basic code modified to log the data:

    var ajax = {};
    ajax.result = Array();
    ajax.fetch = function(urls,datas){
        $.ajax({
            url: urls,
            type: 'post',
            data: datas,
            success: function(data){
                console.log (data);
            }
        });
    }
    ajax.fetch('/notes.html',{});

Here's where the issue lies. If we change the console.log to be a return, and then log the ajax.fetch call, it returns undefined.

Even if I store it in ajax.result and retrieve it through chrome dev. tools, the content appears there, but I'm unable to utilize it in regular javascript.

The page can be found here. It may appear blank, but you can view what it prints out in the console with the provided code above.

javascript ajax

Answer 1

Answer №1

ajax operates as a deferred call by its very nature, rendering the return value from the function insignificant as the initial fetch request has long since completed and returned undefined...

To address this issue, you can integrate a callback function into the fetch method:

var ajax = {};
var result;
    ajax.result = Array();
    ajax.fetch = function(urls,datas, callback){
        return $.ajax({
            url: urls,
            type: 'post',
            data: datas,
            success: function(data){
                callback(data);
            }
        });
    }
    ajax.fetch('/notes.html',{}, function (data) {
        // execute result related code here to ensure variable already assigned
    });

Upon conducting a trace of the operation, the following sequence is observed:

Declaration of ajax object
Declaration of result variable
Assignment of an empty object to ajax
Initialization of ajax.result with a new array (suggest using [] over Array to avoid potential prototype conflicts)
Assignment of the fetch function to ajax.fetch
Invocation of ajax.fetch with an anonymous function acting as the callback parameter
Execution of $.ajax request
Return of ajax.fetch method (leading to the previous undefined outcome, now returning a jQuery deferred object)
Handling of success callback
Invoking the provided callback function
Completion of success callback (unable to capture the return value due to fetch already concluding in step 8)
Usage of done callback (enabling access to fetched data through closures)

Answer 2

ajax operates as a deferred call by its very nature, rendering the return value from the function insignificant as the initial fetch request has long since completed and returned undefined...

To address this issue, you can integrate a callback function into the fetch method:

var ajax = {};
var result;
    ajax.result = Array();
    ajax.fetch = function(urls,datas, callback){
        return $.ajax({
            url: urls,
            type: 'post',
            data: datas,
            success: function(data){
                callback(data);
            }
        });
    }
    ajax.fetch('/notes.html',{}, function (data) {
        // execute result related code here to ensure variable already assigned
    });

Upon conducting a trace of the operation, the following sequence is observed:

Declaration of ajax object
Declaration of result variable
Assignment of an empty object to ajax
Initialization of ajax.result with a new array (suggest using [] over Array to avoid potential prototype conflicts)
Assignment of the fetch function to ajax.fetch
Invocation of ajax.fetch with an anonymous function acting as the callback parameter
Execution of $.ajax request
Return of ajax.fetch method (leading to the previous undefined outcome, now returning a jQuery deferred object)
Handling of success callback
Invoking the provided callback function
Completion of success callback (unable to capture the return value due to fetch already concluding in step 8)
Usage of done callback (enabling access to fetched data through closures)

Answer 3

Answer №2

When using $.ajax(), the method returns immediately after sending the request due to its default asynchronous mode. It's important to note that the return value of the $.ajax call is not the same as the return value of the success function. To enable synchronous behavior, you can modify the code as follows:

ajax.fetch = function(urls, datas)
{
  result = null;
  $.ajax({ url: urls,
           type: 'post',
           data: datas,
           async: false,
           success: function(data) { result = data; } });
  return result;
}

Alternatively, a more concise way to achieve this is:

$.ajax({ url:   urls,
         type:  'post',
         data:  datas,
         async: false }).responseText;

Answer 4

When using $.ajax(), the method returns immediately after sending the request due to its default asynchronous mode. It's important to note that the return value of the $.ajax call is not the same as the return value of the success function. To enable synchronous behavior, you can modify the code as follows:

ajax.fetch = function(urls, datas)
{
  result = null;
  $.ajax({ url: urls,
           type: 'post',
           data: datas,
           async: false,
           success: function(data) { result = data; } });
  return result;
}

Alternatively, a more concise way to achieve this is:

$.ajax({ url:   urls,
         type:  'post',
         data:  datas,
         async: false }).responseText;

Confused about the concept of an Ajax callback? Let's break it down

Answer №1

Answer №2

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