Why is the scope variable lost during the callback?

Question

JavaScript

There is an html page parser that puts data into the DB, with a callback (below). On one page there are several options that must be entered in the table separately. I iterate over them with
$(...).each, then check if the key product_id-option_id exists in the table, updating or writing the new data.

...
"callback":function(error,result,$) {
        if (error) {
            logger.error(error);
            return;
        }
        logger.info('(%d/%d) Url %s %d', this.idx, this.total, this.uri, result.statusCode);
        if (result.statusCode!=200) {
            return;
        }
        // $ is a jQuery instance scoped to the server-side DOM of the page
        var product_new = {
            product_id: this.product_id         
        };        
        // options 
        $("#pr .option").each(function(index,a)                   
                  
                product_new.option_id = $(a).data('opt')||0;        
             
                logger.debug('>> pid=%d, opt=%d',product_new.product_id, product_new.option_id);

                Product.find({product_id: product_new.product_id, option_id: product_new.option_id||0}).success(function(product_old){     
                    logger.debug(index);
                    logger.debug('<< pid=%d, opt=%d',product_new.product_id, product_new.option_id);
                });             
            });  
    }

The problem is that the method (ORM Sequelizejs object) Product.find is passed several pairs of parameters in turn, but in the callback, after searching for them in the database, they are set in all cases to the last pair:

[Friday, January 31, 2014 11:52:59,379] DEBUG >> pid=76, opt=13
[Friday, January 31, 2014 11:52:59,379] DEBUG >> pid=76, opt=308
[Friday, January 31, 2014 11:52:59,380] DEBUG >> pid=76, opt=310
[Friday, January 31, 2014 11:52:59,380] DEBUG >> pid=76, opt=311
[Friday, January 31, 2014 11 :52:59,502] DEBUG 0
[Friday, January 31, 2014 11:52:59,503] DEBUG << pid=76, opt=311
[Friday, January 31, 2014 11:52:59,789] DEBUG 2
[Friday, January 31 , 2014 11:52:59,789] DEBUG << pid=76, opt=311
[Friday, January 31, 2014 11:52:59,793] DEBUG 3
[Friday, January 31, 2014 11:52:59,793] DEBUG << pid =76, opt=311
[Friday, January 31, 2014 11:52:59,802] DEBUG 1
[Friday, January 31, 2014 11:52:59,802] DEBUG << pid=76, opt=311

Moreover, the enumeration index variable is transferred to the one that was when calling the Product.find method, that is, normally. I sinned that product_new is not in the scope because it is defined above by enumeration of options, but if you copy a local variable for each option product_new, it will be the same, but index is normally transferred!

Answer 1

For example, just do at the beginning of each()

var product_id = product_new.product_id,
    option_id = product_new.option_id;

and continue to use them.

Answer 2

callback is called once -> there is only one context -> product_new is defined in it -> shared by all nested functions
anonymous from each() is called for each element -> every time its own context in which index is defined -> anonymous from success() has access to that index, which is defined in the same context as it