How does indirect eval work

Question

I saw on the Internet that people using following construction to get Global object

(1,eval)('this')

or this

(0||eval)('this')

Could you explain how exactly does it work and the benefit over window, top, etc.?

UPD: testing direct vs. indirect eval calls: http://kangax.github.io/jstests/indirect-eval-testsuite/

For me - nothing is wrong, but I want to know what is the point of doing this. For me - it is a black magic without any profit. — hazzik, Sep 19 '13 at 04:27
http://stackoverflow.com/questions/9107240/1-evalthis-vs-evalthis-in-javascript — TGH, Sep 19 '13 at 04:28
I had never heard of this construct prior to reading this question, but I guess you learn something new every day :-) — TGH, Sep 19 '13 at 04:47
To answer your last question, window and top only work on browsers. — bfavaretto, Sep 19 '13 at 04:55
bfavaretto, I know. But I meant other non-magic versions, like pass `window` or `global` to IIFE — hazzik, Sep 19 '13 at 05:48
http://perfectionkills.com/global-eval-what-are-the-options/ should leave no open questions. — Bergi, Sep 19 '13 at 05:58

score 2 · Answer 1 · answered Sep 19 '13 at 07:51

(1,eval)('this') is equivalent to eval('this')

(0||eval)('this') is equivalent to eval('this')

So (1, eval) or (0 || eval) is an expression which yields eval

Like in:

var x = 2;
console.log( (10000, x) );  // will print 2 because it yields the second variable
console.log( (x, 10000) );  // will print 10000 because it yields the second literal
console.log( (10000 || x) ); // will print 10000 because it comes first in an OR
                             // expression

The only catch here it that an object returned from an expression is always the object having the most global scope.

Check that code:

x = 1;

function Outer() {
    var x = 2;
    console.log((1, eval)('x')); //Will print 1
    console.log(eval('x')); //Will print 2
    function Inner() {
        var x = 3;
        console.log((1, eval)('x')); //Will print 1
        console.log(eval('x')); //Will print 3
    }
    Inner();
}

Outer();

How does indirect eval work

1 Answers1