# Array index to balance sums in JavaScript

## Problem

We are required to write a JavaScript function that takes in an array of integers, arr, as the first and the only argument.

Our function is required to pick and return one such index from the array such that the sum of elements on its left side is equal to the sum of elements on its right side. If there exists no such index in the array, we should return -1.

For example, if the input to the function is −

Input

const arr = [1, 2, 3, 4, 3, 2, 1];

Output

const output = 3;

Output Explanation

Because the sum of elements at either side of index 3 is equal (6).

## Example

Following is the code −

Live Demo

const arr = [1, 2, 3, 4, 3, 2, 1];
const balancingIndex = (arr = []) => {
const findSum = arr => arr.reduce((acc, x) => acc + x, 0);
for(let i = 0; i < arr.length; i++){
const leftSum = findSum(arr.slice(0, i));
const rightSum = findSum(arr.slice(i + 1));
if(leftSum === rightSum){
return i;
};
};
return -1;
};
console.log(balancingIndex(arr));

## Output

3

Updated on: 22-Apr-2021

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