从回调访问“ this”范围变量

Question

I am a javascript newbie, i have a problem with the following code. Am trying to set a price value from callback, but isn't working... Any help please?

PreInvoiceSchema.pre('save', function(next) {

//Seperating code from state
var codestate = addr[2].split(/[ ,]+/);
this.customer.zipcode = codestate[2];
this.customer.state = codestate[1];

//Calculating base price
if (this.order_subtype == "Upgrade") {
    this.price = 30;
} else {
    this.price = 50;
}
var self = this;

for (var j = 0; j < this.work.length; j++) {

    Price.findOne({
        "item": self.work[j].workproduct
    }, function(err, val) {
        if (self.work[j] != undefined) {
             self.work[j].workprice = 300; <-- this doesn't work
        }
    });
});

Answer 1

if i'm not mistaken, the closure gets the last value of j. so if the iteration runs from 0 to 3 then you always have self.work[3].workprice = 300; (try tracing).

try working with a library li lodash and turn you code into something like this:

_.each(this.work, function(item) {
    Price.findOne(
        { "item": item.workproduct }, 
        function(err, val) {
            // item.workprice ...
        }
    );
});

also, if this is the real code then it's missing a '}' before the last ')'.

Answer 2

for (var j = 0; j < this.work.length; j++) {
    (function(i) {
        Price.findOne({
            "item": self.work[i].workproduct
        }, function(err, val) {
            if (self.work[i] != undefined) {
                 self.work[i].workprice = 300; <-- this doesn't work
            }
        });
    })(j);
});