const getEquilibrium = (array: number[]): number | null => {
    let postfixSum = array.reduce((acc, curr) => acc + curr, 0);
    let prefixSum = 0;

    for (let i = 0; i <= array.length; i++) {
        const element = array[i];
        postfixSum -= element;

        if (prefixSum === postfixSum) {
            return i;
        }

        prefixSum += element;
    }

    return null;
};

const array = [1, 2, 3, 4, 1, 5];
console.log(getEquilibrium(array));

// Results:
//  [1, 2, 3, 4, 1, 5] -> 3
//  [1, 2, 3, 4, 2, 5] -> null
//  [1] -> 0
//  [] -> null
//  [1, 0] -> 0