function getEquilibriumIndex(arr) {
let sum_arr = 0;
let left_part_sum = 0;
const size = arr.length;
for (let i = 0 ; i < size; i++) {
sum_arr += arr[i];
}
for (let i = 0; i < size; i++) {
sum_arr -= arr[i];
if (left_part_sum == sum_arr) {
return i;
}
left_part_sum += arr[i];
}
return -1;
}
const arr = [-9, 2, 5, 8, -7, 4, 1];
console.log("equilibrium index = " + getEquilibriumIndex(arr));
/*
run:
"equilibrium index = 3"
*/