Finding the missing element in a non-sorted array of consecutive numbers

findMissingElement.js

module.exports = (array, lowerBound, upperBound) => {
    var partialSum = ((array.length + 1) * (lowerBound + upperBound)) / 2;
    var sum = array.reduce((acc, i) => { return acc + i; }, 0);
    return partialSum - sum;
};