Javascript – use redcuce to calculate total cost in array

I have an array with items. Each of these items have the following values

invoice_rows: [
  { item: '', qty: '', price: '' }
],

For each invoice item there is a row added to the array but I want to get the total cost of all these items. So let’s do some math, I have the following array with data

invoice_rows: [
  { item: 'item1', qty: '4', price: '10' }
  { item: 'item2', qty: '2', price: '10' }
  { item: 'item3', qty: '5', price: '5' }
],

The total cost should be the sum of the qty * price for each row. Making the total 40 + 20 + 25 = 85. How can I do this with the reduce method?

>Solution :

Simply calculate qty * price for both items, then add them:

const invoice_rows = [
  { item: 'item1', qty: '4', price: '10' }
  { item: 'item2', qty: '2', price: '10' }
  { item: 'item3', qty: '5', price: '5' }
];
const total = invoice_rows.reduce((a, b) => a.qty * a.price + b.qty * b.price);

using map for this might be slightly more readable, first calculating qty * price for all items, then adding them:

const total = invoice_rows.map(item => item.qty * item.price).reduce((a, b) => a + b);

note that JS will implicitly cast '4' * '10' to the the number 40.