1const arr = [1, 2, 3, 4];
2const sum = arr.reduce((a, b) => a + b, 0);
3// sum = 10

1const arr = [1,2,3,4,5]
2const sum = arr.reduce((a,b) => a+b)

1function getArraySum(a){
2    var total=0;
3    for(var i in a) { 
4        total += a[i];
5    }
6    return total;
7}
8
9var payChecks = [123,155,134, 205, 105]; 
10var weeklyPay= getArraySum(payChecks); //sums up to 722

1// If you do not want to use array.reduece you can itereate through the array and then get the solution.
2const  getSum = (arr)=>{
3  let sum = 0;
4  for(key of arr){
5    sum += key
6  }
7  return sum
8}
9

1[1, 2, 3, 4].reduce((a, b) => a + b, 0)