javascript - Why is the object being iterated passed back through the callback? -

this common idiom encapsualted function undescore.

_.each(obj1, function(val, key, context, obj2){ });

what purpose of 4th argument in callback - obj2. object 1 looping through should available in outer scope obj1.

is there example of need access obj1 through obj2.

i'm trying understand obj2 for. here exact line of code - line 83.

if (iterator.call(context, obj[i], i, obj) === breaker) return;

if passing reference named function callback, obj1 might not available:

function my_callback(val, key, context, obj2) {     ... }  function do_stuff() {     var obj1 = ...     _.each(obj1, my_callback); }

Brande