Reduce 1

 
let vals = [1,2,3,4,5];
let sum = 0;
// let acc represent sum
...
for (let i=0; i < vals.length; i++) {
    sum += vals[i];
}
 
...
for (let val of vals) {
    sum += val;
}
...
 
function sum(acc, val) {
    return acc + val;
}
let answer = vals.reduce(sum);
console.log(sum)