Using a variable as part of a javascript object property

Another very simple question.

I have some code like this:

var a = {"foo":"bar"};

Which creates an object called a with a property called foo with value bar.

I'd like to do this:

var propName = "foo";
a = {propName:"bar"};

And for th开发者_高级运维e two a objects to be the same.

How do I do that?

(Context is that I'm building an object to pass to jQuery's css method)

You must set it in two steps:

var a = {};
a[propName] = 'bar';