array-like/reduce

/**
 * Sum.
 *
 * @param {ArrayLike} a
 * @param {number} i
 * @param {number} j
 * @param {number|bigint} zero
 * @return {number|bigint}
 */
const sum = (a, i, j, zero = 0) => {
    if (i >= j) {
        return zero;
    }

    let total = a[i];

    for (++i; i < j; ++i) {
        total += a[i];
    }

    return total;
};

export default sum;