Need lexical context in eval for Javascript

I am writing a string parser that takes input like "{x} + 20" and evaluates the lexical value of x with the sum of 20 and returns. So

var x = 10;
var new = eval("{x} + 20".replace("/\{(\w+\}/g", "$1"));

and here "new" should equal 30.

How can this be done?

I believe you just do this:

var x = 10
var y = eval("x + 20")

No?