Passing arguments in anonymous functions in JavaScript

Question

Sometimes I see JavaScript that is written with an argument provided that already has a set value or is an object with methods. Take this jQuery example for instance:

Answer 1

Let's take a look at $.each for the example:

each: function (obj, callback, args) {
    var value,
    i = 0,
        length = obj.length,
        isArray = isArraylike(obj);

    if (args) {
        if (isArray) {
            for (; i < length; i++) {
                value = callback.apply(obj[i], args);

                if (value === false) {
                    break;
                }
            }
        } else {
            for (i in obj) {
                value = callback.apply(obj[i], args);

                if (value === false) {
                    break;
                }
            }
        }

        // A special, fast, case for the most common use of each
    } else {
        if (isArray) {
            for (; i < length; i++) {
                value = callback.call(obj[i], i, obj[i]);

                if (value === false) {
                    break;
                }
            }
        } else {
            for (i in obj) {
                value = callback.call(obj[i], i, obj[i]);

                if (value === false) {
                    break;
                }
            }
        }
    }

    return obj;
}

This gets called from

$(".selector").children().each(function(i) {
    console.log(i);
});

like:

return $.each.call(this, callback /* this is your function */, args /* which is an additional thing people rarely use*/ )

This is the line (in the first block) you want to look at

callback.call(obj[i], i, obj[i]);

It's calling the callback, and passing the object as the context – this is why you can use this in the loop. Then the iteration i and then the same object as the context are both sent as arguments to the callback. It's a little bit like magic; until you look at the source code.