How do I interpolate a variable as a key in a JavaScript object?

Question

How can I use the value of the variable a as a key to lookup a property? I want to be able to say: b[\"whatever\"] and have this return 20:

Answer 1

To show all options, I want mention the CoffeeScript way of doing this, which is:

var a = "whatever";
var b = (
  obj = {},
  obj["" + a] = 20,
  obj
);

alert(b.whatever); // 20

Although I prefer:

var a = "whatever";
var b = {};
b[a] = 20;

alert(b.whatever); // 20