cache a function and its return value

CachingFunction.js

var memo = function (fn) {
    var cache = {},
    toType = function(obj) {
      return ({}).toString.call(obj).match(/\s([a-z|A-Z]+)/)[1].toLowerCase();
    };
    return function () {
        var key = [].join.call(arguments, '§') + '§';
        if (!(key in cache)) {
            cache[key] = fn.apply(this, arguments);            
        }
        if (toType(cache[key]) === "array") {
            return cache[key].slice();   
        } else {
            return cache[key];
        }
    };              
};

/*
// ### Side effect ###
var foo = 1;
function bar(baz) {
    return baz + foo;
}
var cached = memo(foo);
cached(1); //2
foo += 1;
cached(1); //2
*/

/*
function loop(count) {
    var x = 0;
    while(count > 0){
        x++;
        count--;
    }
    console.log("done x=%o", x);
    return x;
}

function loop2(count) {
    var x = 0;
    while(count > 0){
        x += 2;
        count--;
    }
    console.log("done x2=%o", x);
    return x;
}

var cachedLoop = memo(loop);
var cachedLoop2 = memo(loop2);
var loopCount = 10e6;

console.time("1st run");
console.log("loops: %o",cachedLoop(loopCount));
console.timeEnd("1st run");

console.log("----------");

console.time("2nd run");
console.log("loops: %o", cachedLoop(loopCount));
console.timeEnd("2nd run");

console.log("----------");

console.time("3nd run - different args");
console.log("loops: %o", cachedLoop(loopCount + 1));
console.timeEnd("3nd run - different args");

console.log("----------");

console.time("4th run");
console.log("loops: %o", cachedLoop2(loopCount));
console.timeEnd("4th run");

console.log("----------");

console.time("5th run");
console.log("loops: %o", cachedLoop2(loopCount));
console.timeEnd("5th run");
*/