CODEWITHSUNDEEP
javascriptunderscore.js
user1637281
user1637281

Reputation:

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

This is a common idiom encapsualted into a function by undescore.

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

What is the purpose of the 4th argument in the callback - obj2. This is the object one is looping through so it should always be available in the outer scope as obj1.

Is there an example of where you would need access to obj1 through obj2.

I'm trying to understand what obj2 is for. Here is the exact line of code - line 83.

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

Upvotes: 0

Views: 42

Answers (2)

T.J. Crowder
T.J. Crowder

Reputation: 1074674

What is the purpose of the 4th argument in the callback - obj2. This is the object one is looping through so it should always be available in the outer scope as obj1.

It's so you can reuse the same function for looping through multiple objects, and still have access to the object being iterated within the function. The function you use might well be define elsewhere. You could have a library of functions you use for various iteration purposes. It's rare to want to know what the object is (hence it being so far into the argument list), but there are use cases.

Upvotes: 1

RichieHindle
RichieHindle

Reputation: 281605

If you were passing a reference to a named function as the callback, obj1 might not be available:

function my_callback(val, key, context, obj2) {
    ...
}

function do_stuff() {
    var obj1 = ...
    _.each(obj1, my_callback);
}

Upvotes: 4

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