javascript - how to get data from JSON variable using a JSON type index -

given:

var data = {   "aaa":[{"firstname":"john", "lastname":"deo"},          {"firstname":"ann", "lastname":"deo"}],   "bbb":[{"name":"john", "age":19},            "name":"jim", "age":21}]  };  var a="aaa"; 

if write this

  data.aaa[1].firstname 

it works. but, how same result using a instead of aaa? like

data.a[1].firstname 

which fails.

when use variable holds key object (a in case) should use square bracket syntax, so:

data[a][1].firstname