arr = [4,5,7,2,6,35,734,574,573,573,57,37,357,357,3,573,7,37]
sum = 0
for (let elem of arr) sum += elem
hs = sum / 2 // half sum
fs = 0 // first sum
for (let i = 0; i < arr.length; i++) {
    fs += arr[i]
    if (fs > hs) {
        console.log(i)
        break
    }
}