function test1(array) {

    for (let i = 0; i < array.length; i++) {

        let lsum = array.slice(0, i).reduce((carry, value) => carry + value, 0);

        let rsum = array.slice(i).reduce((carry, value) => carry + value, 0);

        if (lsum === rsum) return i;

    }

    return -1;

}

​

function test2(array) {

    let sum = array.reduce((carry, value) => carry + value, 0);

    for (let i = 0, lsum = 0, rsum = sum; i < array.length; i++, lsum += array[i - 1], rsum -= array[i - 1]) {

        if (lsum === rsum) {

            return i;

        }

    }

    return -1;

}

​