Array Iteration Interview Problem

gistfile1.js

var _ = require("underscore")._;

// No mutation version
function increment_where_pure(array, match, n) {
    var count;

    function iterator(f, acc, item) {
        // f is the function used to combine array arguments -- push || unshift
        if (count === 0) {
            f.call(acc, item);
        } else {
            if (item === match) { item += 1; }
            f.call(acc, item);
            count -= 1;
        }
        return acc;
    }

    if (n === 0) {
        return _.map(array, function(item) {
            return item === match ? item + 1 : item;
        });
    }

    if (n > 0) {
        count = n + 1;
        return _.reduce(array, iterator.bind(null, Array.prototype.push), []);
    }

    if (n < 0) {
        count = -n + 1;
        return _.reduceRight(array, iterator.bind(null, Array.prototype.unshift), []);
    }
}