The Ajax function is unable to accept JSON data as input

I am trying to figure out why I am unable to access data from a JSON object (json.projects[i].projName) when calling it from within an AJAX function. Below is the code that demonstrates this issue:

var json = JSON.parse(data);

for (var i = 0; i < json.projects.length; i++) {  

alert(json.projects[i].projName); // This will display the correct data

$.ajax({ 
    url: epridlist, 
    method: 'GET'
}).then(function (datas) {
    alert(json.projects[i].projName); // This call will fail and report that projName is not known, even though projName exists in the JSON data
});
}

I would appreciate any assistance in understanding this issue. The result displays correctly if I alert the value outside of the AJAX function.

Answer №1

One of the reasons behind this issue is closure. The inner function within the Ajax success callback creates a closure, capturing the value of variable i. This inner function holds a reference to the i variable.

Once the loop finishes its execution, the value of i becomes json.projects.length. Attempting to access

json.projects[json.projects.length]
will result in receiving undefined.

An effective solution to this problem would be to consider using the let variable instead of var, as it offers a narrower scope compared to function scope.

You may explore alternative methods by referring to this question: JavaScript closure inside loops – simple practical example

var json = JSON.parse(data);

for (let i = 0; i < json.projects.length; i++) {  

alert(json.projects[i].projName); //will display correct data

    $.ajax({ url: epridlist, method: 'GET'}).then(function (datas) {
        alert(json.projects[i].projName) // this operation will fail, indicating an unknown projName, even though projName exists in the JSON data
    });
}

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