Get an object´s property by its name using a string

I have read quite often how to call a function using a string in JavaScript. But how can I get the property of an object on the same way? This post demonstrates how to use strings in order to call functions. How can I realize the same thing with object properties?

I tried this: Fiddle

But this line does not give me the desired result.

alert( window[val] ); 

What am I doing wrong ?

That´s the full code:

f1("obj.key"); 

function f1(val){
   var obj={
      key : "Hello World"
   };
   alert( obj.key );       // Hello World
   alert( window[val] );   // undefined
}