Retrieve Value From Multi-dim Associative Array Using Only One String As The Key?

Lets assume the following object

Code:
var x = {
y: {
z: 10
}
} ;

Next, I need to retrieve the value '10', but the only thing I have is one string as a key For example (this doesn't work):

Code:
var str = '[y][z]' ; // or 'y.z'??
alert(x[str]) ; // should alert 10

Can this be done using a single string as input for x ?

UPDATE: a solution which comes close to what I was looking for is

Code:
var str = "x['y']['z']" ;
alert(eval(str));