Incorrect variable value retrieved in AJAX callback

Question

Incorrect variable value retrieved in AJAX callback

I am working on a code that utilizes JSON to validate VAT numbers.

My main goal is to identify which VAT numbers are valid

BTW[0] = 'NL1234567890';
BTW[1] = 'NL1233537891';
BTW[2] = 'NL1232346894';

var arraylength = BTW.length;

for (var i = 0; i < arraylength; i++) {
  
 var BTWnummer = BTW[i];
 
 callUrl = 'http://isvat.appspot.com/'+Land+'/'+BTWnummer+'/?callback=?';
 
 $.getJSON(callUrl, BTWnummer, function(data){
 alert(data+' '+BTWnummer);
 
 });
 
}

The data variable provides a true or false response. However, I am facing a challenge in obtaining the correct BTWnummer within the JSON function. It seems to always retain one BTW number. I suspect that the JSON process is asynchronous, so I am seeking guidance on how to access the accurate number within the JSON code snippet. Based on my experimentation, it does utilize different numbers in the callUrl.

javascript json

Answer 1

Answer №1

The issue arises when the variable BTWnummer is modified by the time the callback is triggered due to the loop being fully executed before the asynchronous callbacks.

To overcome this, you can preserve its value by using an immediately invoked function:

for (var i = 0; i < arraylength; i++) {
   (function(BTWnummer){
      var callUrl = 'http://isvat.appspot.com/'+Land+'/'+BTWnummer+'/?callback=?';
      $.getJSON(callUrl, BTWnummer, function(data){
         alert(data+' '+BTWnummer);
      });
   })(BTW[i]);
}

If readability is a concern, you can also achieve the same result using a named function instead of an anonymous one:

function f(BTWnummer){
  var callUrl = 'http://isvat.appspot.com/'+Land+'/'+BTWnummer+'/?callback=?';
  $.getJSON(callUrl, BTWnummer, function(data){
     alert(data+' '+BTWnummer);
  });
}
for (var i = 0; i < arraylength; i++) {
    f(BTW[i]);
}

This approach works because the scope of a variable in JavaScript is limited to the function execution. Each execution of f maintains a distinct value for BTWnummer (research "closure" for more information).

With the upcoming ES6 version, the need for this workaround will be eliminated as the let keyword will introduce block-scoped variables.

Answer 2

The issue arises when the variable BTWnummer is modified by the time the callback is triggered due to the loop being fully executed before the asynchronous callbacks.

To overcome this, you can preserve its value by using an immediately invoked function:

for (var i = 0; i < arraylength; i++) {
   (function(BTWnummer){
      var callUrl = 'http://isvat.appspot.com/'+Land+'/'+BTWnummer+'/?callback=?';
      $.getJSON(callUrl, BTWnummer, function(data){
         alert(data+' '+BTWnummer);
      });
   })(BTW[i]);
}

If readability is a concern, you can also achieve the same result using a named function instead of an anonymous one:

function f(BTWnummer){
  var callUrl = 'http://isvat.appspot.com/'+Land+'/'+BTWnummer+'/?callback=?';
  $.getJSON(callUrl, BTWnummer, function(data){
     alert(data+' '+BTWnummer);
  });
}
for (var i = 0; i < arraylength; i++) {
    f(BTW[i]);
}

This approach works because the scope of a variable in JavaScript is limited to the function execution. Each execution of f maintains a distinct value for BTWnummer (research "closure" for more information).

With the upcoming ES6 version, the need for this workaround will be eliminated as the let keyword will introduce block-scoped variables.

Incorrect variable value retrieved in AJAX callback

Answer №1

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