List box value is not populated is not reflected in code behind with JavaScript

I have a listbox that I am populating at runtime with javascript. After the listbox is populated I am using a button to get that value. on button click when I am trying to get the value it is the listbox item count is 0, Can anyone tell what's wrong
after t he javascript populates the code,I have written the code in button click :
  for (int i = 0; listbox1.Items.Count - 1; i++)
   {
       string s = listbox1.Items[i].ToString();
   }
the value of count is showing as 0, though it has the values populated .
What I am missing ?
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