How can I detect I'm inside an eval() call?

Question

Does there exist a string s such that

(new Function(s))();

and

eval(s);

behave differently? I\'

Answer 1

The current answer does not work in strict mode since you can't redefine eval. Moreover, redefining eval is problematic for many other reasons.

The way to differenciate them is based on the fact that well... one of them creates a function and what doesn't. What can functions do? They can return stuff :)

We can simply exploit that and do something with return:

// is in function
try {
     return true;
} catch(e) { // in JS you can catch syntax errors
     false; //eval returns the return of the expression.
}

So in example:

var s = "try{ return true; }catch(e){ false; }";
eval(s); // false
Function(s)(); // true
(new Function(s))(); // true, same as line above
(function(){ return eval(s); })(); // the nested 'problematic' case - false