[danik.js]

var array = ['x', 'y', 'z'],
    expression = "x+y+z";
 
var scope = new Scope(array);
 
var result = eval('with(scope){' + expression + '}');
alert(result);

function Scope(array){
   this[array[0]] = 1;   // x=1
   this[array[1]] = 2;   // y=2
   this[array[2]] = 3;   // z=3
}