[рони]

platedz,
Вариант ...

var num = 2
var object_1 = {a: "a1",b:"b1"}
var object_2 = {a: "a2",b:"b2"}
var object_3 = {a: "a3",b:"b3"}
var obj = ['',object_1,object_2,object_3]
alert(obj[num]['a']);