How to sum the values of an array

Short answer

The reduce() method adds the elements up, starting from zero:

console.log([10, 20, 30].reduce((sum, n) => sum + n, 0));

Output:

Divide the sum by the array length. The check prevents division by zero:

const marks = [4, 5, 3];
const sum = marks.reduce((total, n) => total + n, 0);
 
console.log(marks.length > 0 ? sum / marks.length : 0);

Output:

Access the property you need inside the function:

const orders = [{ total: 100 }, { total: 250 }];
 
console.log(orders.reduce((sum, order) => sum + order.total, 0));

Output:

The same with an ordinary for...of loop:

let sum = 0;
for (const n of [10, 20, 30]) sum += n;
 
console.log(sum);

Output:

Numbers from forms and files often come as strings. Without converting them, + glues the strings together:

const values = ['10', '20', '30'];
 
console.log(values.reduce((sum, n) => sum + n, 0));
console.log(values.reduce((sum, n) => sum + Number(n), 0));

Output:

0102030
60