(null,eval)('')

nulleval.js

//Came across this in the es-discuss list, on the "clean scope" thread:
https://mail.mozilla.org/pipermail/es-discuss/2011-August/thread.html#16294

//I do not understand what the "null," part buys.
//Has to do something with scope(?), but at least in Firebug, 
//I can see foo inside the string being evaled.

var foo = 'foo';
(null,eval)('(function(){console.log(foo);}())');

//Prints "foo" in Firebug console.

Also, if you define foo in the global scope and then use an indirect eval foo will still be in scope. That's another reason why you want to wrap those 2 statements in a function.

Thanks all for the answers.

It's absolutely about eval, not about browser quirks. When you write

eval(/* blah */)

or even

(((((eval)))))(/* blah */)

it's specified in the standard to be a "direct eval" which has access to the local scope. But when you write

(null, eval)(/* blah */)

it's an "indirect eval," which is evaluated in the global scope.

The null isn't particularly special; it would also work to replace the null with 42 or /someregex/ or "this is a srsly weird language feature".

Dave

+1 for @kangax's definitive source on global eval.

http://perfectionkills.com/global-eval-what-are-the-options/#indirect_eval_call_theory

ah... i learn something new everyday ... :)

so its about whether the reference is named or not
eval() would execute in local scope, but any other reference to eval would execute in global scope

the following execute in a global scope

• (null, eval)('')
• (x=eval)('')
• var x=eval; x('')
• var eval=window.eval; eval('')
• window.eval('')
• eval.call(window,'')

edit: and i found this http://perfectionkills.com/global-eval-what-are-the-options/#indirect_eval_call_examples :)