Question

Does there exist a string s such that

(new Function(s))();

and

eval(s);

behave differently? I’m trying to “detect” how a string is being evaluated.

Answer 1

Check for the arguments object. If it exists, you’re in the function. If it doesn’t it has been evaled.

Note that you’ll have to put the check for arguments in a try...catch block like this:

var s = 'try {document.writeln(arguments ? "Function" : "Eval") } catch(e) { document.writeln("Eval!") }';
(new Function(s))();
eval(s);

Demo

Solution to nnnnnn‘s concern. For this, I’ve edited the eval function itself:

var _eval = eval;
eval = function (){
    // Your custom code here, for when it's eval
    _eval.apply(this, arguments);
};

function test(x){
    eval("try{ alert(arguments[0]) } catch(e){ alert('Eval detected!'); }");
}
test("In eval, but it wasn't detected");​