Session makes the variable update data twice - angularjs

Question

Every time I get a new price, the following is supposed to happen:
totalprice = totalprice + cart[i]
For example, if the totalprice was previously 500 and a new price is 800, the totalprice is supposed to be (500 + 800) whereas it's currently (500 + 500 + 800). Why does this happen?
exports.add = function(req, res) {
var cart = req.body.cart;
console.log('frome node..adding checkcart.js');
console.log("Session Initialized!!");
for(var i = 0 ; i < cart.length ; i++) {
totalprice = cart[i].price + totalprice;
console.log('Current Price: '+cart[i].price);
}
req.session.cart = cart;
console.log(cart);
console.log('total: ' + totalprice);
//req.session.totalprice = totalprice;
json_responses = {
"totalprice": req.session.totalprice
}
res.send(json_responses);
};

Answer 1

This is one of many answers. I would suggest creating a variable that will count the amount of items already added. Otherwise, your for loop will keep counting them all again and again. Bare in mind your question lacks a lot of code, so this will have me add two options.
Option 1
Every time a user adds an item the add function will be called. For this option all you have to do is add the last added item to the total price calculation. So in your code, instead of:
for(var i = 0 ; i < cart.length ; i++) {
totalprice = cart[i].price + totalprice;
console.log('Current Price: '+cart[i].price);
}
Use:
totalprice = cart[cart.length-1].price + totalprice;
Option 2
This option is for when a user can add many items, and the prices are calculated once the user clicks a button that calls the add method. This is just slightly more tricky. You need to add a counter that will count the items already calculated. So in code:
var counter = 0;
exports.add = function(req, res) {
var cart = req.body.cart;
var prevCounter = counter;
counter = req.body.cart.length - counter; // items added in the session
console.log('frome node..adding checkcart.js');
console.log("Session Initialized!!");
for(var i = prevCounter ; i < counter ; i++) {
totalprice = cart[i].price + totalprice;
console.log('Current Price: ' + cart[i].price);
}
req.session.cart = cart;
console.log(cart);
console.log('total: ' + totalprice);
//req.session.totalprice = totalprice;
json_responses = {
"totalprice": req.session.totalprice
}
res.send(json_responses);
};
Good luck !