Find the missing number

missing_number.js

/* You've probably seen those goofy comments in forums that say "find the
 * missing number in a list of 10,000 numbers. Who wants to take time to
 * do that? I know there are arithmetical solutions to this problem, but
 * I thought I'd find a simple functional approach to this.
 *
 * This snippet simply takes the array of numbers (expected to be unsorted,
 * but either will work), sorts them, then uses the reduce function to compare
 * to the previous value in the array. I like this method more than summing
 * all numbers, dividing by two, comparing, blah, blah. The map function works
 * great!
 */
[1, 5, 3, 2].sort(function(a, b) {
    return a - b;
}).reduce(function(previous, current) {
    current !== previous + 1 && console.log(current - 1);
    return current;
});