Pass Function Parameter in Ajax Data Call

Question

Title may not be correct, tried my best to explain the issue. Spent hours trying to figure this, and I can't.

I basically have a JS function with an AJAX call within it.

function doAjax(input) {
     $.ajax({
          url: "validate.php?input=" + input,
          data: $("form").serialize(),
          dataType: 'json',
          type: 'POST',
          async: false,
          success: function (data) {
               alert(data.input);
          }
      });
}

I want to alert data from json based on the input parameter.

alert(data.(THIS SHOULD BE THE FUNCTION PARAMETER));

Currently, it returns undefined because data.input isn't defined.

EG: If the function is called like this: doAjax("username");

The alert should be data.username.

I hope this makes sense, and I hope someone can help. Thanks!