Reverse a string or array using JavaScript.

reverse.js

'use strict';

/**
 * Reverses a string or array.
 * 
 * @param {(String|Array)} i - The string or array.
 * @return {(String|Array)}
 */
function reverse(i) {
    var isString = false;
    if (typeof i === 'string') {
        isString = true;
    } else if (i.constructor !== Array) {
        return null;
    }

    var result = [];
    for (var n = 0, len = i.length; n < len; n++) {
        result.unshift(
            isString ? i.slice(n, n + 1) : i.slice(n, n + 1)[0]
        );
    }
    return isString ? result.join('') : result;
}

if (typeof module !== 'undefined' && module.exports) {
    module.exports = reverse;
}

// tests
console.assert(reverse('foobar') === 'raboof');
console.assert(reverse('') === '');
console.assert(reverse([1, 3, 3, 7]).toString() === '7,3,3,1');
console.assert(JSON.stringify(reverse([])) === '[]');
console.assert(reverse({}) === null);
console.assert(reverse(42) === null);

// benchmark
console.time('reverse');
Array.apply({}, { length: 10000 }).forEach(function() {
    reverse('lorem ipsum dolor sit amet');
});
console.timeEnd('reverse');