memoify 1 argument function. this has no criteria to limit cache size. be careful.

memo.js

/*
 * single argument memo where argument cached as string (no objects please)
 *
 * @function memo
 * @param  {Function} fn function to memoize
 * @return {Function} memoized version of the function
 */
var memo = function(fn) {
    return function(a) {
        if (!fn._memo) {
            fn._memo = {};
        }
        var v;
        if (a in fn._memo) {
            v = fn._memo[a];
        }
        else {
            v = fn(a);
            fn._memo[a] = v;
        }
        return v;
    };
};