gistfile1.js

//calling test1 will **not** define a function you can call in the global scope
//functions defined in eval are scoped to the function they're called from
function test1()
{
    eval('function boo(){alert("No one can call me.")}');
}
boo(); //boo() is not defined error

//calling test2 **will** define a function you can call in the global scope
//the window object and the global scope are the same things in javascript
function test(){
    eval('window.boo = function (){alert("Boo! Did I scare you?")}');
};
boo(); //alert window will show