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The ajax call you are doing inside the loop is asynchronous, this means that when the execution reaches the $.ajax[...] line, it will run "in the background" while the normal execution continues through the loop.

What is most likely happening in your code is that the execution will reach the alert(avl_qty) line before the ajax responses from the loop reach you. You can test this by letting the script run. You'll see that it will execute the alert("before avilability ajax"), then alert(avl_qty) and finally all the alert(JSON.stringify(data)); from the ajax requests.

To solve the issue you will have to wait for the async calls to finish, you can use async/await or even callbacks, here's an example.