Vue.js How to calculate totals?

var payments = new Vue({
            el: "#payments",
            data: {
                payments: [
                    { name: "houseRent", amount: 1000, is_paid: true },
                    { name: "houseRent", amount: 1500, is_paid: true },
                    { name: "houseRent", amount: 1200, is_paid: false },
                    { name: "houseRent", amount: 1070, is_paid: true },
                    { name: "houseRent", amount: 1040, is_paid: false }
                ]
            },
            computed: {
                totalAmount: function () {
                    var sum = 0;
                    this.payments.forEach(e => {
                        sum += e.amount;
                    });
                    return sum
                }
            }
        });`

As you proposed, you could use the Array#reduce function. Starting from this example on SO, you could adapt it to your needs and only add value.total to the sumtotal.

To compute the total of all values, you can use computed properties, which will display as {{ total }} in your template:

<script>

export default {



    props: {
        values: {
            type: Array,
            default: []
        },
    }
    ready: function() {

    },
    computed: {
        total: function() {
            if (!this.values) {
                return 0;
            }

            return this.values.reduce(function (total, value) {
                return total + Number(value.total);
            }, 0);
        }
    }

}
</script>

Note: This will of course only work, if value.total is a unitless number (e.g. 1, not '1 USD'). Otherwise you would need to strip the in the reduce function as well.

In case anyone else is in the same situation as me I thought I'd add this answer. I needed to get the values from nested objects then push them to an array before reducing them:

total: function(){

  let total = [];

  Object.entries(this.orders).forEach(([key, val]) => {
      total.push(val.price) // the value of the current key.
  });

  return total.reduce(function(total, num){ return total + num }, 0);

}

This uses ES7 .entries to loop through the object which looked like this:

orders = {
    1: {title: 'Google Pixel', price: 3000},      
    2: {title: 'Samsung Galaxy S8', price: 2500},
    3: {title: 'iPhone 7', price: 5000}
  }

You can then display the total in your template with:

<span> {{total}} </span>