[jQuery] ajax question

if we want to transfer some variable to another page by ajax, we use
data: {a: 1, b: 2}
and it will send like xxx.php?a=1&b=2
but if a is not a constant variable, such as
var $a = x
data: {$a: 1} I want it become xxx.php?x=1
var $a = y
data: {$a: 1} I want it become xxx.php?y=1
how to do that?